Algorithm::Combinatorics:

#! perl -slw
use strict;
use Algorithm::Combinatorics qw[ combinations combinations_with_repeti
+tion ];

our $K //= 3;
our $ALPHA //= join '', 'a'..'d';

my @alpha = split '', $ALPHA;

print "$K from [@alpha] without repetition";
my $iter = combinations( \@alpha, $K );
print "@$_" while $_= $iter->next;

print "$K from [@alpha] with repetition";
$iter = combinations_with_repetition( \@alpha, $K );
print "@$_" while $_= $iter->next;

__END__
C:\test>1117899
3 from [a b c d] without repetition
a b c
a b d
a c d
b c d
3 from [a b c d] with repetition
a a a
a a b
a a c
a a d
a b b
a b c
a b d
a c c
a c d
a d d
b b b
b b c
b b d
b c c
b c d
b d d
c c c
c c d
c d d
d d d
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