It's still best to avoid gratuitous use of $a and $b.

When $a and $b are declared properly, no conflict. But if you forget to declare them, you can quietly get action at a distance.

#! perl -slw
use strict;

( $a, $b ) = ( 'mya', 'myb' );
# imagine several lines of intervening code
{
    our( $a, $b ) = ( 'oura', 'ourb' );
    my @sorted = sort { $a <=> $b } map{ rand 100 } 0 .. 99;
    print "$a : $b";
}
print "$a : $b";
__END__
C:\s>perl sortab.pl
oura : ourb
oura : ourb #no warnings, $a and $b are quietly changed
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The script dies if they are declared (with my instead of our) in the same scope as the sort routine:

#! perl -slw
use strict;

my( $a, $b ) = ( 'mya', 'myb' );

{
    my( $a, $b ) = ( 'oura', 'ourb' );
    my @sorted = sort { $a <=> $b } map{ rand 100 } 0 .. 99;
    print "$a : $b";
}
print "$a : $b";
__END__
C:\S\pl>perl sortab.pl
Can't use "my $a" in sort comparison at sortab.pl line 10.
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Update: Deleted extra code tag