chromatic has explicated a problem with each, which is both an array and hash operator. He details it in Inadvertent Inconsistencies: each in Perl 5.12 and Inadvertent Inconsistencies: each versus Autoderef. In short, if you use it with a reference, Perl doesn’t know until it actually executes the each if it’s going to use it’s array or hash behavior (and in some cases, blow up with either). However, as the programmer, I probably know which behavior I want:

while( my( $index, $value ) = each $ref ) { my $elem = $other_array->[$index]; } # I want array behavior
while( my( $key, $value ) = each $ref ) { ... } # I want hash behavior

The problem isn’t when it blows up, which is easy to catch (it blows up). If you get the wrong sort of reference, you’ll get nonsensical indices or keys. If you have an array reference, you’ll get numbers with the first return value. If you have a hash reference, you’ll get strings. If you get strings but treat them as array indices, you’ll likely always get array index 0, unless the key is a number. You might even get an odd index. If the key is 123Buster, you’ll get array index 123 due to Perl’s numification. Going the other way, using an array reference when you expected a hash, you’ll have to find keys that are whole numbers.

Effective programs reduce ambiguity in their code, but this new feature increases it. It’s easy to fix; you dereference them yourself. If you have the wrong reference type, you’ll find out right away:

while( my( $index, $value ) = each @$ref ) { my $elem = $other_array->[$index]; } # I want array behavior
while( my( $key, $value ) = each %$ref ) { ... } # I want hash behavior

If you really wanted to keep the auto-dereferencing feature, you could check the reference type before you use it, but what’s the point of saving a character with the auto-dereferencing if you have to wrap the whole thing in a guard condition?

if( ref $ref eq ref [] ) {
    while( my( $index, $value ) = each @$ref ) { ... }
    }

Now keys has the same problem. You can use that either with an array or a hash, but at some point you’re probably going to have to know what sort of reference you have so you can use the key to dereference it. At that point, you effectively declare what sort of reference it should have been. If you have the wrong sort of reference, your script dies:

my $ref = [ ... ];
foreach my $key ( keys $ref ) {
    my $elem = $ref->{$index}; # Big error!
    }

This problem is the unintended consequence of letting the other array and hash operators take a scalar variable as an argument and letting the parser automatically add the bits to dereference. David Golden wanted more magic syntax and the patch wasn’t so tough. To get the nicer syntax in some cases you end up dealing with more special cases. I noted this at the time David proposed it, but his enthusiasm for the interesting parts of the problem steamrolled over the bad parts.

The Unicode::UCD module gives you access to some of the information about a character:

use Unicode::UCD 'charinfo';
use charnames qw(:full);
use Data::Dumper;

my $charinfo   = charinfo(
	ord( "\N{SMILING CAT FACE WITH OPEN MOUTH}" )
	);
print Dumper( $charinfo );

The output has many of the properties, but not all of them:

$VAR1 = {
		  'digit' => '',
		  'bidi' => 'ON',
		  'category' => 'So',
		  'code' => '1F63A',
		  'script' => 'Common',
		  'combining' => 0,
		  'upper' => '',
		  'name' => 'SMILING CAT FACE WITH OPEN MOUTH',
		  'unicode10' => '',
		  'decomposition' => '',
		  'comment' => '',
		  'mirrored' => 'N',
		  'lower' => '',
		  'numeric' => '',
		  'decimal' => '',
		  'title' => '',
		  'block' => 'Emoticons'
		};

This doesn’t include the Age of the character, that is, when the character was added to Unicode. This might seem like a silly thing to know, but it came in handy typesetting Programming Perl. We had problems with some characters but we couldn’t see a pattern until we looked at the age of all the problem characters. Any character added after Unicode 4.0 didn’t typeset correctly. It took some annoying work to get the age by scanning through each age until that property matched:

#!/Users/brian/bin/perls/perl5.15.7

use v5.10;
use utf8;

use List::Util qw(first);

my @chars =  ( 'a', '→', '⣽', "\N{SMILING CAT FACE WITH OPEN MOUTH}" );

my @ages = qw( 1.1 2.1 2.0 3.0 3.1 3.2 4.0 4.1 5.0 5.1 5.2 6.0 );

foreach my $char ( @chars ) {
	my $age = first { $char =~ /\p{Age=$_}/ } @ages;
	say "Age: $age";
	}

It works, but it’s an unsatisifying kludge:

a Age: 1.1
→ Age: 1.1
⣽ Age: 3.0
😺 Age: 6.0

Now, Unicode::UCD has a prop_invmap to create an index based on a property you choose and a _search_invlist to return the offset in the map:

#!/Users/brian/bin/perls/perl5.15.7

use 5.15.7;
use utf8;

use charnames qw(:full);
use List::Util qw(first);
use Unicode::UCD;

my @chars =  ( 'a', '→', '⣽', "\N{SMILING CAT FACE WITH OPEN MOUTH}" );

my @ages = qw( 1.1 2.1 2.0 3.0 3.1 3.2 4.0 4.1 5.0 5.1 5.2 6.0 );

foreach my $char ( @chars ) {
	my $age = age_of_char( $char );
	say "$char Age: $age";
	}

sub age_of_char {
	my( $char ) = @_;
	# create the inverted list, once
	# can only initialize as scalar
	state $inv = _make_age_inverted_list();

	my $i = Unicode::UCD::_search_invlist($inv->[0], ord $char);
	return $inv->[1][$i];
	}

# create the inverted list, once
sub _make_age_inverted_list {
	state( $list, $map, $format, $default, $init );
	unless( $init++ ) {
		($list, $map, $format, $default) = Unicode::UCD::prop_invmap("Age");
		$format eq "s" || die "wrong format $format";
		}
	return [ $list, $map ];
	}

That looks like a lot of work, but most of it happens once to setup the inversion map.

If you grew up in the ASCII world, case insensitivity is a difference of literally one bit, so changing case is setting or unsetting a bit in the octet that represents that character.

If you’ve read the Perl FAQ, you may have seen this quip:

“Perl” is the name of the language. Only the “P” is capitalized. The name of the interpreter (the program which runs the Perl script) is “perl” with a lowercase “p”.

When Larry Wall was asked what the difference between “Perl” and “perl”, he said “One bit”. It’s literally a difference of flipping one bit in the ASCII representation. That’s as complicated as ASCII case folding gets.

The capital letter P has the ordinal value 0b1010000. The small letter p, which shows up later in the ASCII sequence, has the ordinal value 0b1110000. This makes it extremely easy to write routines to change between upper and lower cases:

use v5.10;

say "  U L";
say "-----";

foreach my $char ( qw(p P a b c A B C) ) {
	my $lower = chr( ord($char) | 0b0100000 );
	my $upper = chr( ord($char) & 0b1011111 );

	say "$char $upper $lower";
	}

The output shows what you’d expect for the upper and lower cases:

  U L
-----
p P p
P P p
a A a
b B b
c C c
A A a
B B b
C C c

Since bit flipping is easy to do, it’s very easy for even primitive computers to quickly change case (assuming that you’re not so primitive as to not have two cases). But, this only works if you restrict the output to the ASCII letters. If you want to handle non-letters, you have to do a bit more work to ensure that you don’t shift them into other characters:

use v5.10;

say "  U L";
say "-----";

foreach my $char ( qw(p P a b c A B C # !) ) {
	my $upper = uppercase( $char );
	my $lower = lowercase( $char );

	say "$char $upper $lower";
	}

 sub lowercase {
 	my $_ = shift;
  	my $ord = ord();

 	return $_ unless $ord >= 0x41 and $ord <= 0x5A;
	return chr( $ord ^ 0b100000 );
	}

 sub uppercase {
 	my $_ = shift;
 	my $ord = ord();

 	return $_ unless $ord >= 0x61 and $ord <= 0x7A;
	return chr( $ord ^ 0b100000 );
	}

Now the non-letters stay the same character:

  U L
-----
p P p
P P p
a A a
b B b
c C c
A A a
B B b
C C c
# # #
! ! !

This almost works for Latin-* encodings too. When you move out of the ASCII sequence into Unicode, you don't have this luxury, and it's not merely a representational issue.

If you were infected with ASCII early, you've grown up thinking that you can go back and forth between upper and lower cases and always get the same result. Outside of ASCII, that's not necessarily true. Consider the word "Reichwaldstraße", a common street name in Germany. The "straße" has the special character ß (U+00DF ʟᴀᴛɪɴ ꜱᴍᴀʟʟ ʟᴇᴛᴛᴇʀ ꜱʜᴀʀᴘ ꜱ). which is a ligature of a long s, the fancy ſ (U+017F ʟᴀᴛɪɴ ꜱᴍᴀʟʟ ʟᴇᴛᴛᴇʀ ʟᴏɴɢ ꜱ) that you may have seen in historical documents, and the familiar short s. Put them together, ſs, and move them close enough and you can see how you would end up with ß once you connect the hanging portion of the long s with the top of the short s. The UCS has an uppercase version (U+1E9E ʟᴀᴛɪɴ ᴄᴀᴘᴛɪᴀʟ ʟᴇᴛᴛᴇʀ ꜱʜᴀʀᴘ ꜱ), although no one uses it aside from saying that no one uses it. U+1E9E lowercases to U+00DF, but U+00DF has no single character uppercase version; it's the two characters SS. The lowercase of SS, however, is ss:

use utf8;

my $string = "Reichwaldstraße";

my $upper = uc( $string );
my $lower = lc( $upper  );

print <<"HERE";
Started with: $string
Upper:        $upper
Lower:        $lower
HERE

The output shows that you don't get back to the original:

Started with: Reichwaldstraße
Upper:        REICHWALDSTRASSE
Lower:        reichwaldstrasse

There's another s that causes problems: the Greek sigma, which comes in two lowercase forms. One appears in the middle of words and the other appears at the end, as in όσος, where σ and ς represent the same thing, just in different forms mandated by their position:

use utf8;

my $char = "όσος";

my $upper = uc( $char );
my $lower = lc( $upper );

print <<"HERE";
Started with: $char
Upper:        $upper
Lower:        $lower
HERE

Again, the lowercase version at the end is different than what you started with:

Started with: όσος
Upper:        ΌΣΟΣ
Lower:        όσοσ

This means that you can't merely use lc to normalize text for case insensitive comparison. These won't compare correctly:

lc( "Reichwaldstraße" ) eq lc( "REICHWALDSTRASSE" );  # Nope!
lc( 'όσος' ) eq lc( 'ΌΣΟΣ' );                         # Nope!

You might object that these are different strings and that they shouldn't be the same, but where did these strings start? Perhaps that REICHWALDSTRASSE was not originally all uppercase, but changed by some stupid filters between you and the original information (and with a name like mine, I know about stupid casing filters). That's part of the ASCII infection.

So, lc is the wrong way. Sadly, we do this incorrectly in Learning Perl, when we show this subroutine we want to sort:

sub case_insensitive { "\L$a" cmp "\L$b" }

The Unicode specification solves this with its case folding rules. In short, it folds characters with different case forms into a common form. There's not a rule for this; they do it by exhaustion, specifying the common form for each fold. The common form is defined in the Unicode Character Database, which the Perl developers have digested into the files you find in the unicore/ directory in your Perl library. Here's a few lines from unicore/CaseFolding.txt:

0050; C; 0070; # LATIN CAPITAL LETTER P
0051; C; 0071; # LATIN CAPITAL LETTER Q
0052; C; 0072; # LATIN CAPITAL LETTER R
00DF; F; 0073 0073; # LATIN SMALL LETTER SHARP S
03A3; C; 03C3; # GREEK CAPITAL LETTER SIGMA
03C2; C; 03C3; # GREEK SMALL LETTER FINAL SIGMA
FB00; F; 0066 0066; # LATIN SMALL LIGATURE FF
FB01; F; 0066 0069; # LATIN SMALL LIGATURE FI
FB02; F; 0066 006C; # LATIN SMALL LIGATURE FL
FB03; F; 0066 0066 0069; # LATIN SMALL LIGATURE FFI
FB04; F; 0066 0066 006C; # LATIN SMALL LIGATURE FFL

The first column is the code number of the original character, the second is the type of folding (explained in the data file and coming up later), and the third column are the characters that form the common, folded ("equivalent") version. Essentially, it's a big hash. Notice that some of the folded versions are multiple characters. You're not going to get that with bit fiddling.

Case folding takes the character in the first column and turns them into the characters in the third column, then takes the result and does it again until there are no more folds possible. It keeps doing that until there is nothing to replace. Characters that don't have an entry in this file fold into themselves. You case fold to compare strings, not to normalize strings for storage or other uses. Case folding makes case insensitive comparisons very fast, but it also loses information that you can't recover. You can read the exact rules in Section 5.18, "Case mapping", of the Unicode Standard.

To see how that works, try that with Reichwaldstraße and όσος. All characters except two stay the same, and two use the mapping from unicore/CaseFolding.txt:

• Reichwaldstraße → reichwaldstrasse
• REICHWALDSTRASSE → reichwaldstrasse
• όσος → ΌΣΟΣ
• ΌΣΟΣ → όσοσ

To implement these operations, Perl v5.16 adds the fc built-in function. Instead of lc, use that:

use v5.15.8;  # until we get v5.16  XXX feature
fc( "Reichwaldstraße" ) eq fc( "REICHWALDSTRASSE" );  # Yep!
fc( 'όσος' ) eq fc( 'ΌΣΟΣ' );                         # Yep!

If you don't have v5.16, you can use the fc front the Unicode::CaseFold module on CPAN.

If you wanted to do this inside a double-quoted string, you can use the \F case shift operator (but be aware of the things we noted in Understand the order of operations in double quoted contexts). Our Learning Perl example could change to:

sub case_insensitive { "\F$a" cmp "\F$b" }

More complicated folds

Looking back at the extract of unicore/CaseFolding.txt, you might remember that I skipped over the second column, the mapping status. Those letters stand for different folding rules:

• C: common case folding
• F: full case folding (strings may grow in length)
• S: simple case folding (map to single characters)
• T: special case for uppercase I and dotted uppercase I

The "T" status stands in for folds that the general rules can't handle, mostly some characters from Turkish and similar languages.

So far, Perl's fc only handles the "F" status for full case folding. It doesn't handle the special folding you'll find in unicore/SpecialCasing.txt that has the oddball situations, such as multiple source characters folding onto other multiple characters. If you want to handle those, you're on your own, although the Unicode::Casing module on CPAN might help.

Many of the folding rules depend on the source language, so you'll probably want to pay special attention if you are using that language or completely ignore them if you are not.

Besides that, the Universal Character Set gives people much more of a chance to mess up. Suppose that you want to write "β-carotene", that thing you get from carrots. That first character is β (U+03B2 ɢʀᴇᴇᴋ ꜱᴍᴀʟʟ ʟᴇᴛᴛᴇʀ ʙᴇᴛᴀ). Some people might think it looks like ß (U+00DF ʟᴀᴛɪɴ ꜱᴍᴀʟʟ ʟᴇᴛᴛᴇʀ ꜱʜᴀʀᴘ ꜱ), and that's good enough for them. No amount of case folding is going to let you know that someone used an incorrect character. But, this is also one of the benefits of Unicode: characters know what they are.

Another correct way

There's another correct way to check strings regardless of case. You can use the /i flag on the match operator. The Unicode-aware Perl regex engine handles the rest:

use utf8;
use v5.15.7;

use Set::CrossProduct;

my $string = "Reichwaldstraße";

my $upper = uc( $string );
my $lower = lc( $upper  );

my $sets = Set::CrossProduct->new(
	[
	[ $string, $upper, $lower ],
	[ $string, $upper, $lower ],
	]
	);

foreach my $tuple ( $sets->combinations ) {
	my( $l, $r ) = @$tuple;
	next if $l eq $r;

	say "lc($r) eq lc($l)  ? ", lc($r) eq lc($l) ? "matched" : "failed";
	say "fc($r) eq fc($l)  ? ", fc($r) eq fc($l) ? "matched" : "failed";
	say "$r =~ m/$l/i      ? ", $l =~ m/$r/i ? "matched" : "failed";

	say;
	}

In the output, you can see that lc sometimes fails, but that the fc and m//i always works:

lc(REICHWALDSTRASSE) eq lc(Reichwaldstraße)  → failed
fc(REICHWALDSTRASSE) eq fc(Reichwaldstraße)  → matched
REICHWALDSTRASSE =~ m/Reichwaldstraße/i      → matched

lc(reichwaldstrasse) eq lc(Reichwaldstraße)  → failed
fc(reichwaldstrasse) eq fc(Reichwaldstraße)  → matched
reichwaldstrasse =~ m/Reichwaldstraße/i      → matched

lc(Reichwaldstraße) eq lc(REICHWALDSTRASSE)  → failed
fc(Reichwaldstraße) eq fc(REICHWALDSTRASSE)  → matched
Reichwaldstraße =~ m/REICHWALDSTRASSE/i      → matched

lc(reichwaldstrasse) eq lc(REICHWALDSTRASSE)  → matched
fc(reichwaldstrasse) eq fc(REICHWALDSTRASSE)  → matched
reichwaldstrasse =~ m/REICHWALDSTRASSE/i      → matched

lc(Reichwaldstraße) eq lc(reichwaldstrasse)  → failed
fc(Reichwaldstraße) eq fc(reichwaldstrasse)  → matched
Reichwaldstraße =~ m/reichwaldstrasse/i      → matched

lc(REICHWALDSTRASSE) eq lc(reichwaldstrasse)  → matched
fc(REICHWALDSTRASSE) eq fc(reichwaldstrasse)  → matched
REICHWALDSTRASSE =~ m/reichwaldstrasse/i      → matched

The match operator isn't useful for sort though, since you can only tell if the strings are the same.

Things to remember

• Case-folding is more complicated than merely lowercasing.
• The fc does proper case folding according to the Unicode standard.
• The \F case fold operator does full case folding in double-quoted contexts.

Although __SUB__ looks like __FILE__, __LINE__, and __PACKAGE__, each of which are compile-time directives, the __SUB__ happens at run time so you can use it with subroutines you define later.

First, consider how you’d try to do this without the __SUB__ feature. You could declare a variable to hold a subroutine reference then in a later statement define the subroutine. Since you’ve already declared the variable, you can use it in the definition. Perl won’t de-reference it until you actually run the subroutine, so it doesn’t matter that it’s not a reference yet:

use v5.10;

my $sub;

$sub = sub {
	state $count = 10;
	say $count;
	return if --$count < 0;
	$sub->();
	};

$sub->();

Your output is a countdown:

10
9
8
7
6
5
4
3
2
1
0

To do that, there are two requirements: the code reference must be stored in a variable, and the variable must already be defined. That’s not always convenient. Not only that, your anonymous subroutine contains a reference to itself, so you’d either have to play games with weak references or just let the reference live forever. Neither of those are attractive.

Rafa�l Garcia-Suarez solved these problems by creating Sub::Current to give you a ROUTINE function that returns a reference to the current subroutine, even if it is a named subroutine:

use v5.10;
use Sub::Current;

sub countdown {
	state $count = 10;
	say $count;
	return if --$count < 0;
	ROUTINE->();
	};

countdown();

You might want to define these code references as a single statement, even you don’t need to. This is useful for inline subroutines where you want to define the code reference in the parameter list:

use v5.10;
use Sub::Current;

sub run { $_[0]->() };

run( sub {
		state $count = 10;
		say $count;
		return if --$count < 0;
		ROUTINE->();
		}
	);

You may want to define the subroutine in one statement as a return value:

use v5.10;
use Sub::Current;

sub factory {
	my $start = shift;
	sub {
		state $count = $start;
		say $count;
		return if --$count < 0;
		ROUTINE->();
		}
	};

factory(4)->();

Using this module has the disadvantage of a CPAN dependency, although a very light one because it’s self contained. There’s another module, Devel::Caller, from Richard Clamp that can can get a code reference from any level in the call stack, including the current level:

use v5.10;
use Devel::Caller qw(caller_cv);

sub factory {
	my $start = shift;
	sub {
		state $count = $start;
		say $count;
		return if --$count < 0;
		caller_cv(0)->();
		}
	};

factory(7)->();

Perl 5.16 lets you do the same thing without the CPAN module:

use v5.15.6;  # until v5.16 is released

sub factory {
	my $start = shift;
	sub {
		state $count = $start;
		say $count;
		return if --$count < 0;
		__SUB__->();
		}
	};

As with many new features added since Perl v5.10, you can enable __SUB__ with a use VERSION statement,
as you see in the previous example, or with the feature pragma and the current_sub import:

use feature qw(say state current_sub);

sub factory {
	my $start = shift;
	sub {
		state $count = $start;
		say $count;
		return if --$count < 0;
		__SUB__->();
		}
	};

factory(7)->();

Things to remember

• Perl v5.16 provides the __SUB__ directive to return a reference to the currently running subroutine
• Import this new feature by requiring the Perl version or through
  the feature pragma

• Prior to Perl v5.16, you can do this the same thing with Sub::Current

A double-quoted string has three features:

• Variable interpolation
• Escaped and logical characters
• Case shift operators

You might have missed this because the documentation doesn’t emphasize it. There is a single sentence in perlop, but in relation to the regular expression operators and the \Q:

For double-quoted strings, the quoting from \Q is applied after interpolation and escapes are processed.

If you don’t pay attention to the order of these operations, you’ll get results that you might not expect. The problem is that the order of operations isn’t the same in all double-quoted contexts.

In strings, the order of operations is the same as listed earlier:

• Variable interpolation
• Escaped and logical characters
• Case shift operators

Variable interpolation

You already know about variable interpolation. This is one of Perl’s greatest features, and the one I miss the most when I have to use a different language:

my $cat = 'Roscoe';
my $string = "Buster $cat Mimi";

In a double quoted context, Perl substitutes the value of $cat. You end up with Buster Roscoe Mimi.

Case-shift operators

The case-shift operators change parts of a double-quoted string. Although we call them “case shift”, not all of them change the case.

The \F and fc are new for the yet unreleased Perl v5.16. Those will show up in a different Item. Notice there’s no \f for a fcfirst. That double-quoted sequence already means “form feed”, the instruction to printers to stop the current page and start a new page.

Look at some examples using these in a double-quoted string:

% perl -e 'print "\ubuster\n"'
Buster
% perl -e 'print "\LBUSTER\n"'
buster
% perl -e 'print "\Ubuster\n"'
BUSTER
% perl -e 'print "\Ubus\Eter\n"'
BUSter
% perl -e 'print "\LBUST\EER\n"'
bustER
% perl -e 'print "\QP*rl\n"'
P\*rl\

That last one is a bit odd. It looks like it ends with a \. It doesn’t really end like that because there’s a newline that \Q quoted:

% perl -e 'print "\QP*rl\n"' | hexdump -C
00000000  50 5c 2a 72 6c 5c 0a                 |P\*rl\.|
00000007

Perl handled the “\n” before it handled the \Q, but the meta-character quoter thinks the newline is a special character so it escapes it. An escaped newline is just a newline, though.

Now, combine these with variable interpolation. Perl handles the variables first then does the case shifting:

use 5.14.1;

my $cat = 'Buster';

say "Roscoe $cat Mimi";
say "Roscoe \U$cat Mimi";
say "Roscoe \U$cat\E Mimi";

The results are probably not surprising. The first line is just interpolation, the second line uppercases everything from \U to the end, and the third line uppercases only the parts between the \U and the \E:

Roscoe Buster Mimi
Roscoe BUSTER MIMI
Roscoe BUSTER Mimi

If the case shift happens after interpolation, you might think that you could interpolate a case shift:

use 5.14.1;

my $cat = '\UBuster'; # no case shift in a single quote!

say "Roscoe $cat Mimi";

That doesn’t work though. The intended case shift operator shows up as literal characters because Perl doesn’t do double processing:

Roscoe \UBuster Mimi

A \U inside the string doesn’t bother the escaped characters because Perl has already processed those:

use 5.14.1;

my $cat = 'Buster';

say "Roscoe \U$cat\a\n Mimi";

The “\n” is still a newline and the “\a” is still the bell, and everything after the \U is uppercased (if it has an uppercase equivalent).

That seems simple enough. It’s variable interpolation followed by character escapes followed by case shifting. But this is Perl, so it can’t be that easy.

Regular expression double quoting

The regular expression operators (qr, m//, and s///) handle the double quote operations differently. From perlop:

For the pattern of regex operators (qr//, m// and s///), the quoting from \Q is applied after interpolation is processed, but before escapes are processed.

Now the order is of operations is:

• Variable interpolation
• Case shift operators
• Escaped and logical characters

You can see this when you print the stringified forms of the patterns:

% perl -le 'print qr/\Q\n/'
(?-xism:\\n)
% perl -le 'print qr/\U\n/'
(?-xism:\N)

You probably expect all of these to match, but not all of them do:

% perl -le 'print "\n" =~ qr/\n/ ? "Yes" : "No"'
Yes
% perl -le 'print "\n" =~ qr/\Q\n/ ? "Yes" : "No"'
No
% perl -le 'print "\n" =~ qr/\U\n/ ? "Yes" : "No"'
No
% perl -le 'print "\n" =~ qr/\l\n/ ? "Yes" : "No"'
Yes
% perl -le 'print "\n" =~ qr/\L\n/ ? "Yes" : "No"'
Yes

The last two times are curious. The \l and \L leave the n as a lowercase n so in the last step, the \n is still a newline. Those two tests still match.

This means that you can construct a string and a pattern with the same sequence of characters, but they might not match:

% perl -le 'print "\Q\n" =~ qr/\Q\n/ ? "Yes" : "No"'
No
% perl -le 'print "\U\n" =~ qr/\U\n/ ? "Yes" : "No"'
No
% perl -le 'print "\L\n" =~ qr/\L\n/ ? "Yes" : "No"'
Yes

It’s even worse. What does the \N mean? It depends on the Perl version:

% perl5.10.1 -le 'print "\n" =~ qr/\N/ ? "Yes" : "No"'
Missing braces on \N{} in regex; marked by <-- HERE in m/\N <-- HERE / at -e line 1.
% perl5.12.1 -le 'print "\n" =~ qr/\N/ ? "Yes" : "No"'
No
% perl5.14.1 -le 'print "\n" =~ qr/\N/ ? "Yes" : "No"'
No

Perl v5.12 added \N as "not a newline" to replace the . no matter which default regex switches you have. That's why Perl v5.10 thinks you have an incomplete \N{CHARNAME}. The others match a newline because the case shift happens in the middle of the process:

% perl5.10.1 -le 'print "\n" =~ qr/\L\N/ ? "Yes" : "No"'
Yes
% perl5.14.1 -le 'print "\n" =~ qr/\L\N/ ? "Yes" : "No"'
Yes
% perl5.8.9 -le 'print "\n" =~ qr/\L\N/ ? "Yes" : "No"'
Missing braces on \N{} at -e line 1, near "\L"
Execution of -e aborted due to compilation errors.

With the \N{CHARNAME} syntax, you can match characters by their name in the Universal Character Set. Here you match an uppercase A:

% perl -Mcharnames=:full -le 'print "A" =~ qr/\N{LATIN CAPITAL LETTER A}/ ? "Yes" : "No"'
Yes

If you put a \L in front of that, you might think it would match the lowercase version of the named letter. There's no such luck because the \L affects the pattern before the \N{CHARNAME}:

% perl -Mcharnames=:full -le 'print "A" =~ qr/\L\N{LATIN CAPITAL LETTER A}/ ? "Yes" : "No"'
No
% perl -Mcharnames=:full -le 'print "a" =~ qr/\L\N{LATIN CAPITAL LETTER A}/ ? "Yes" : "No"'
No

The \N turns into a newline and the braces are now for a quantifier with a non-number in it:

% perl -Mcharnames=:full -le 'print qr/\L\N{LATIN CAPITAL LETTER A}/'
(?-xism:\n{u+41})

You might think that this would match anything since that should probably turn into \n{0} just like the values in the array index turn into integers. The perlre section on "Quantifiers" don't say what should happen, but if it's not a number, the braces become literals. Here's a simple demonstration that those braces are literals:

% perl -le 'print "\n{a}" =~ qr/\n{a}/ ? "Yes" : "No"'
Yes

Here's the pattern you created before, and that you want to match now:

% perl -Mcharnames=:full -le 'print qr/\L\N{LATIN CAPITAL LETTER A}/'
(?^u:\n{u+41})

It doesn't match a lowercase a:

$ perl -Mcharnames=:full -le 'print "a" =~ qr/\L\N{LATIN CAPITAL LETTER A}/ ? "Yes" : "No"'
No

It doesn't match a newline either. The pattern in \n{u+41} and that's not a quantifier. There are some characters after the \n, so the target string doesn't have enough characters to match:

% perl -Mcharnames=:full -le 'print "\n" =~ qr/\L\N{LATIN CAPITAL LETTER A}/ ? "Yes" : "No"'
No

Using the regular expression text doesn't work either, which you might miss on the first pass:

% perl -Mcharnames=:full -le 'print "\n{u+41}" =~ qr/\L\N{LATIN CAPITAL LETTER A}/ ? "Yes" : "No"'
No

Of course! That + is a quantifier, so it isn't a literal character that should show up in the string. So this works:

% perl -Mcharnames=:full -le 'print "\n{u41}" =~ qr/\L\N{LATIN CAPITAL LETTER A}/ ? "Yes" : "No"'
Yes

This works too because you can have one or more of u:

% perl -Mcharnames=:full -le 'print "\n{uuuuu41}" =~ qr/\L\N{LATIN CAPITAL LETTER A}/ ? "Yes" : "No"'
Yes

If you don't want the \L to extend into character name sequence, you can use the \E to limit its effect:

% perl -Mcharnames=:full -le 'print "bar" =~ qr/\LB\E\N{LATIN SMALL LETTER A}r/ ? "Yes" : "No"'
Yes

Things to remember

• The double quote string constructor handles variable interpolation, special characters, and case shift operators in that order.
• The regular expression operators handles variable interpolation, case shift operators, and special characters in that order.
• Double-quoted interpolation in a match operator happens before regular expression compilation.
• The min-max quantifier is only a quantifier if you give it numbers. Otherwise, it's literal characters.