#!/usr/bin/perl -l

use strict;
use warnings;

{
    my @a=qw/a b c d e f/;
    my %order;
    for (0..$#a) {
        $order{ $a[$_] } = $_ + 1;
    }

    sub idx { $order{$_[0]} || 0 }
    sub order {
        sort { idx($a) <=> idx($b) } @_;
    }
}

$,=$";
print +(order qw/f d b/);

__END__