use strict;
use warnings;

sub permutation {
    my ($perm,@set) = @_;
    unless(@set) {
      print "$perm\n" || die("Print error:$!");
    }
    else {
      permutation($perm.$set[$_], (@set[0..$_-1],@set[$_+1..$#set]) )
            foreach (0..$#set);
    }
    return;
}

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