#!/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";
}

##</code><code>##

        ( sub { [ ( @_ ? $_ : 0 ) .. $#chars ] } ) x $k

##</code><code>##

 $ comb 10 10 | wc -l # with replacement
92378
 $ comb -10 19 | wc -l # without replacement
92378

##</code><code>##

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

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

@ARGV = ( 3, 'a'..'e' )
    if  ! @ARGV;
my( $k, @chars ) = @ARGV;
if( 1 < @chars ) {
} elsif( $chars[0] =~ /^[1-9][0-9]*$/ ) {
    @chars = 1..$chars[0];
} else {
    @chars = $chars[0] =~ /./g;
}

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