Chain comparisons to avoid excessive typing

Checking that a value is between two others involves two comparisons, and so far in Perl that’s meant that you’ve had to type one of the values more than once. That gets simpler in v5.32 with chained comparisons. This would make Perl one of the few languages that support the feature. So far its implemented in v5.31.10 and until v5.32 is actually released, it isn’t a real feature.

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Unquoted empty heredoc terminators are now fatal

Continuing its quest to clean up long deprecated features, v5.28 takes care of another feature deprecated since v5.0. You can no longer neglect to specify a heredoc separator. This was a warning in v5.26 and is now fatal. You probably weren’t doing this anyway (I’ve never seen it in the wild), but it’s nice to know the edge cases are disappearing.

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Perl v5.22 adds hexadecimal floating point literals

You can specify literal hexadecimal floating-point numbers in v5.22, just as you can in C99, Java, Ruby, and other languages do. Perl, which uses doubles to store floating-point numbers, can represent a limited set of values. Up to now, you’ve had to specify those floating point numbers in decimal, hoping that a double could exactly represent that number. That hope, sometimes unfounded, is the basis for the common newbie question about floating point errors. Continue reading “Perl v5.22 adds hexadecimal floating point literals”

Intercept warnings with a __WARN__ handler

Perl defines two internal pseudo-signals that you can trap. There’s one for die, which I covered in Override die with END or CORE::GLOBAL::die and eventually told you not to use. There’s also one for warn that’s quite safe to use when you need to intercept warnings. Continue reading “Intercept warnings with a __WARN__ handler”

Use for() instead of given()

[Lexical $_ was removed in v5.24]

Perl 5.10 introduced the given-when feature, a fancier version of the C switch feature. However, it was poorly designed and tested and depended on two other dubious features, the lexical $_ and smart-matching. Parts of this feature are salvageable, but you should avoid the literal given (and probably the lexical $_ and the smart matching, but I’ll skip those for this Item). Continue reading “Use for() instead of given()”

Understand autovivification

Perl will autovivify complex data structures when you use them as if they already exist. This feature saves you a lot of annoying work defining structures that you intend to use. However, this also means that Perl might create data structures that you don’t intend to use in code that isn’t just assigning values. Continue reading “Understand autovivification”

Know the two different forms of eval

Perl’s eval leads a double life, and, like Dr. Jekyl and Mr. Hyde, one is dangerous and one is almost safe. And, it’s important to know which one is dangerous; I grew up thinking that Dr. Jekyl was the bad one because evil people, such as Dr. No, had titles.

You can recognize the evals by their first, and only, argument. One form takes a string and the other takes a block. The string version compiles a string as Perl code and executes its, all at runtime. The block form runs, at run time, code that perl has already compiled. Continue reading “Know the two different forms of eval”