Of course, one can skirt the issue in version 5.6+ using prototypes because then the arguments arrive in @_ as with ordinary subroutines (though it will be slower than the package variable method):

#!/usr/bin/perl -w
use strict;

package Foo;
sub numeric1 ($$)   {
    my($a, $b) = @_;  # could use any variable names
    $a <=> $b
}

# or just
sub numeric2 ($$) {$_[0] <=> $_[1]}

package main;
my($a, $b) = (13, 42);
my @array   = (3, 2, 5, 3, 4, 1);
my @sorted1 = sort Foo::numeric1 @array;
print "@sorted1\n";
my @sorted2 = sort Foo::numeric2 @array;
print "@sorted2\n";
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