While it might be cool that you use variables without declaration, but if that's what you want then just go without use strict; and then don't be surprised that few people will be ready to help you fixing your code.

So assuming you want the effect of use strict that requires you to declare every variable before use, then you might notice it won't require the declaration of $a or $b.

In other words, this code will work:

use strict;
use warnings;

$a = 23;
$b = 19;
print $a+$b, "\n";   # 42

The problem is not really in the use of these variable outside of sort and the other special cases. They won't be even changed by a call to sort as they are going to be localized for the duration of sort as you can see here:

use strict;
use warnings;

$a = 23;
$b = 19;

print $a+$b, "\n";    # 42

my @numbers = sort {$a <=> $b } 1, 3, 2;
print "@numbers\n";  # 1 2 3 
print $a+$b, "\n";   # 42

It's just the confusion they might cause to someone who is not familiar with their special behavior, and the bad example we provide to others.

Therefore a long time ago I've decided that I won't use $a or $b even in short snippets or sample code. If I want to use short and meaningless variables names, there are plenty of other letters in the abc.

BTW Not using $a and $b is one of the suggestions if you'd like to improve your code.