It is better to avoid using special variables '$a' and '$b'.
Use different name, e.g. '$_a', or longer meaningful name, or uppercase letter.

#!/usr/bin/perl -lw

use strict;

my ($a, $b) = (7, 5);
($b, $a) = ($a, $b);

print join ' ', sort { $a <=> $b } qw( 5 28 1 );

print "$b $a";
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STDOUT:

5 1 28
7 5
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STDERR:

"my $a" used in sort comparison at prog.pl line 8.
"my $b" used in sort comparison at prog.pl line 8.
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I suggest to update examples with using of different names ;)