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

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

our $level=0;

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

    local $level = $level+1;
    print " "x$level . "$level:$perm\n";



    print "$perm\n" || return unless (@set);

    permutation($perm.$set[$_],@set[0..$_-1],@set[$_+1..$#set]) foreach (0..$#set);

}

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

permutation('',@input);