Why doesn't Perl warn about using `$a` outside of sort block anymore?

Question

I wrote some program for Perl 5.26.1, using strict and warnings. So after some tests without a message I thought it's OK.

But when I ran the program wth older Perl 5.18.2, I suddenly got a warning about $main::a being used only once. Indeed, there was a mis-spelling where $a should have been $an actually.

I know that $a and $b have special semantics when used in a "sort block", but I wonder why in Perl 5.26.1 use of $a seems to be OK even outside such a block (My use was in a print statement within a function and some "else branch" within a nested loop).

The manual page (perlvar) says:

Special package variables when using "sort()", see "sort" in perlfunc. Because of this specialness $a and $b don't need to be declared (using "use vars", or "our()") even when using the "strict 'vars'" pragma. Don't lexicalize them with "my $a" or "my $b" if you want to be able to use them in the "sort()" comparison block or function.

So is there a rationale behind not warning the user, or is it a bug?

Dave Mitchell · Accepted Answer · 2025-07-04 13:07:00Z

10

From the perl-5.20.0 delta:

=head2 $a and $b warnings exemption

The special variables $a and $b, used in C<sort>, are now exempt from "used
once" warnings, even where C<sort> is not used.  This makes it easier for
CPAN modules to provide functions using $a and $b for similar purposes.
[perl #120462]

So it was an intentional change.

answered Jul 4 at 13:07

Dave Mitchell

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5 Comments

ikegami Jul 6 at 1:32

e.g. List::Util's reduce

U. Windl Jul 7 at 6:18

Could you add references or examples where using $a or $b is important "to provide functions using $a and $b for similar purposes"? I thought $a and $b are some ancient low-level optimizations buried in the Perl core.

Dave Mitchell Jul 8 at 7:13

The standard way that Perl's built-in sort function works is that it temporarily sets $a and $b to the two values to be compared before calling the compare function. So it makes sense not to warn for $a and $b . As ikegami has pointed out, other people have written functions such as List::Util's reduce(), which use a similar pattern - i.e. the reduce method repeatedly sets $a and $b before calling the supplied block..

U. Windl Jul 8 at 8:08

So maybe the perlvar manual page should add a comment stating that normal user programs should avoid using variables $a and $b.

Dave Mitchell Jul 8 at 13:10

$a and $b are already mentioned in perlvar.

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