Uhm... well forget it, the code is crap. This one does the right thing:

sub idx {
  my $lvl = shift;                 # ARG1: get level
  my @out = qw(a1 b1 c1 d1 a2 b2 c2 d2 a3 b3 c3 d3 a4 b4 c4 d4);
# init OUT array

  return \@out if($lvl == 1);      # bail out if lvl = 1

  my @tmp;

  for my $o (@out) {
    push @tmp,  map { "$o$_" } @{&idx($lvl-1)};
  }

  return \@tmp;
}
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Probably I should not do my coding sunday morning. Marcel