Update: (especially for those who didn't like this post) sauoq explained in greater detail the potential issues with using $a and $b as general purpose variables. He also pointed out that, as is well known, in most cases it won't do much harm. However it was apparent that the person I was answering to was not aware of them and the OP appeared to be a newbie. So, in this context, I'm still convinced it was an important circumstance to bring to their knowledge. Sad to notice more than one's mileage does vary...

$a should not be used as a general purpose variable

It's probably better to explain that rather than refer to a manpage that won't. At least, not very well. Yes, sort uses package variable $a and $b. It also localizes them.

$ perl -le '$a="foo"; my @n = (3,2,1); print for sort {$a<=>$b} @n; pr
+int $a'
1
2
3
foo
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It only becomes an issue if you declare them as lexical variables.

$ perl -le 'my $a; my @n = (3,2,1); print for sort {$a<=>$b} @n'
Can't use "my $a" in sort comparison at -e line 1.
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And the better fix is probably not to avoid $a and $b but to be explicit in your sort blocks and subs by using $::a and $::b (or $Foo::a and $Foo::b if you are in package Foo) explicitly.

$ perl -le 'my $a; my @n = (3,2,1); print for sort {$::a<=>$::b} @n;'
1
2
3
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That isn't to say avoiding $a and $b is a bad thing... they are generally lousy variable names anyway. But I often use them in one-liners. So long as you know when and, more importantly, why it can be an issue, there's no harm in it.

-sauoq
"My two cents aren't worth a dime.";

That isn't to say avoiding $a and $b is a bad thing... they are generally lousy variable names anyway. But I often use them in one-liners.

Me too. Also in golf/obfu when I want to keep the code strict safe (even if there would be other built in vars available and provided that it's easy to do so), like in this one.