#!/usr/bin/perl
use strict;
use warnings;

sub permutation {
    my ($perm, $depth, @set) = @_;
    print "\t" x $depth, "@set\n";
    print "$perm\n" unless @set;
    permutation($perm . $set[$_],
                $depth + 1,
                @set[ 0      .. $_ - 1],
                @set[ $_ + 1 .. $#set]
    ) for 0 .. $#set;
}

my @input = (qw/a b c d/);
permutation("", 1, @input);