use strict;
use warnings;

my @input = (qw/a b c d/);

permutation('', @input);

sub permutation {
    my ($perm, @set) = @_;
    
    if (!@set) {
        print "$perm\n";
        return;
    }
        
    for my $partition (0 .. $#set) {
        my $firstEnd = $partition - 1;
        my $lastStart = $partition + 1;
        my @newSet = @set[0 .. $firstEnd];
        
        push @newSet, @set[$lastStart .. $#set];
        permutation($perm . $set[$partition], @newSet);
    }
}