Category Archives: miscellany

Undef a scalar to release its memory

When you store a large string in a scalar, perl allocates the memory to store that string and associate it with the scalar. It uses the same memory even if you assign a much shorter value to the same scalar. Use the functional form of undef to let perl reuse that memory for something else. […]

Perl v5.28 can delete key-value slices

Perl v5.20 introduced key-value slices that worked on hashes and arrays. You could extract values by their keys or indices as well as assigning to those. The key-value slice delete is way to extract the keys and values you want and delete them at the same time. You can destructively

Perl v5.26 now recognizes version control conflict markers

Perl v5.26 can now detect and warn you about a version control conflict markers in your code. In prior versions, the compiler would try to interpret those as code and would complain about a syntax error. You program still fails to complain but you get a better error message. Maybe some future Perl will bifurcate […]

In-place editing gets safer in v5.28

In-place editing is getting much safer in v5.28. Before that, in rare circumstances it could lose data. You may have never noticed the problem and even with all the times I’ve explained it in a Perl class I haven’t really thought about it. This was first reported as early as December 2002 and after we […]

Beware of the removal of when in Perl v5.28

[Although I haven’t seen an official notice besides a git commit that reverts the changes, by popular outcry these changes won’t be in v5.28. It’s not that they won’t happen but they won’t be in v5.28. People who depend on Perl should stay vigilant. My advice in the first paragraph stands—change is coming and we […]

keys in scalar context now returns the number of keys

Starting in v5.26, a hash in scalar context evaluates to the number of keys in the hash. You might have thought that it always did that just like an array (not a list!) in scalar context evaluates to the number of items. But nope—it evaluated to a seemingly useless number called the “hash statistics”. Now […]

Apple recommends installing your own perl

Apple recommends installing your own perl (or python or ruby) for your private development to not interfere with the work the bundled perl (or python or ruby) does. In Item 110. Compile and install your own perls. we recommended the same thing.

Make bitwise operators always use numeric context

[This feature is no longer experimental, starting in v5.28. Declaring use 5.28 automatically enables them.] Most Perl operators force their context on the values. For example, the numeric addition operator, +, forces its values to be numbers. To “add” strings, you use a separate operator, the string concatenation operator, . (which looks odd at the […]

Use a computed label with loop controllers

Not sure which loop you want to break out of? Perl v5.18 makes that easy with computed labels. The value you give next, last, and redo no longer has to be a literal. You could already do this with goto, but now you can give the loop controllers an expression.

In v5.20, -F implies -a implies -n

Perl was once known for its one-liners in its sysadmin heydays. People would pass around lists of these one liners, many of which replaced complicated pipelines that glued together various unix utilities to do some impressive system maintenance.