#https://perlmonks.org/?node_id=11124854

use strict;
use warnings;
use Data::Dump qw/pp dd/;
use 5.10.0;


sub permutation {
    my ($perm,@set) = @_;

    my $level  = 5 - @set;
    my $marker = " "x$level . "$level: " ;
    say "$marker ENTER perm='$perm' set=(@set)";

    unless (@set) {
        say "$marker RESULT: $perm";
    } else {
        permutation( $perm.$set[$_],
                     @set[0..$_-1], @set[$_+1..$#set]
                   ) for (0..$#set);
    }

    say "$marker RETURN";
    return;
}

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

permutation('',@input);


my @set = @input;
my $perm = "";

for (0..3) {                            # level 1
    my @set =@set;
    my $perm = $perm . splice @set,$_,1;
    for (0..2) {                        # level 2
        my @set =@set;
        my $perm = $perm . splice @set,$_,1;
        for (0..1) {                    # level 3
            my @set =@set;
            my $perm = $perm . splice @set,$_,1;
            for (0..0) {                # level 4
                my @set =@set;
                my $perm = $perm . splice @set,$_,1;

                say $perm;              # level 5
            }
        }
    }
}