Well, lets say you dont want to use a global variable, suppose your binary tree is composed of hash elements with "left" and "right" keys (where left<=mid<right) you might try something like this:

sub find_key_dupes {
  my $binary_tree=shift;
  my $key=shift;
  my $sub;
  my @dupes;
  $sub=sub {
              my $node=shift;
              return unless $node;
              if ($node->{key} lt $key) {
                  $sub->($node->{right});
              } else {
                  push @dupes,$node if $node->{key} eq $key;
                  $sub->($node->{left});
              }
           };
  $sub->($binary_tree);
  return \@dupes;
}
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This is using closures to simulate the effect of a Pascal's nested subroutines. Its a nice trick for having lexically scoped vars acting as globals to a recursive subroutine. This avoids having to pass them as parameters. Also, my guess is that you could avoid recursion entirely and do this with one sub (without nesting) using iterative techniques, which would be faster but probably slightly more complicated code.

Oh and I whipped that code out pretty fast, so its untested but you get the idea I hope. (And I hope its correct ;-)

Yves / DeMerphq
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Writing a good benchmark isnt as easy as it might look.