#!/usr/bin/perl -l
use strict;
use Algorithm::Loops 'NestedLoops';

sub pick {
    my( $k, @chars ) = @_;
    my $idx = NestedLoops( [
        ( sub { [ ( @_ ? 1+$_ : 0 ) .. @chars-$k+@_ ] } ) x $k
    ] );
    return sub { @chars[ $idx->() ] };
}

@ARGV = ( 3, 'a'..'e' )
    if  ! @ARGV;

my $iter = pick( @ARGV );
while( my @subset = $iter->() ) {
    print "@subset";
}