Update: I missed the part about needing arbitrarily sized tuples. This only groups in sets of two...my bad. :(

Original message follows...

Ah, the joys of dot products. No need for recursion:

#!/usr/bin/perl -w

@set = qw(A B C);

print join("\n", map(join(',', @$_), @{&make_tuples(\@set, \@set)}));

sub make_tuples {
  my($set1, $set2) = @_;
  my @product;
  foreach my $x (0 .. $#$set1) {
    foreach my $y (0 .. $#$set2) {
      push(@product, [$set1->[$x], $set2->[$y]]);
    }
  }
  \@product;
}
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I daresay I just answered a homework question for you, but so be it.

Matt