use Data::Dumper;

use strict;
use warnings;

my @set = ('1', '2', '3', '4');
my $len = 3; #test shows that it is safe to specify len greater than set size, or equal to set size, or less than
my @strings = generate($len, @set);
print Dumper(\@strings);

sub generate {
    my $len = shift;
    my @set = @_;
    my @strings;
    if ($#set == 0) {
        return ($set[0]);
    } else {
	if ($len == 1) {
	    push @strings, "$_" foreach (@set);	
	} else {
            for (0 .. $#set) {
                my @temp = @set;
                my $t = splice(@temp, $_, 1);
	        push @strings, "$t$_" foreach (generate($len - 1, @temp));
	    }
	}
        return @strings;
    }
}