$ cat foo.pl
use strict;
use warnings;

my @list = (qw(now is the time));

sub compare {
    my ($left, $right) = @_;
    print "Comparing $left and $right.\n";
    return  1 if $left gt $right;
    return -1 if $left lt $right;
    return  0;
}

print "BEFORE: ", join(" ", @list), "!\n";

@list = sort { compare($a,$b) } @list;

print "AFTER: ", join(" ", @list), "?\n";

$ perl foo.pl
BEFORE: now is the time!
Comparing now and is.
Comparing the and time.
Comparing is and the.
Comparing the and now.
AFTER: is now the time?