I'd consider that bad use of a global variable. Whether you are using OO or not. There are perfectly valid uses for globals, but to pass data in and out of functions isn't one of them.

A few things to consider:

• What if during you query an exception is thrown? Then everything that catches an exception has to take care of cleaning up the global.
• What if you have a more complex operation, and a query might actually do another query on the tree? With a global, your algorithm wouldn't be "re-entrant".

There's no good reason to want to use a global here, it's trivial in Perl to combine lists. Just use recursion and return a list of answers. Here's an example for doing a range query on an ordered binary tree:

sub range_query {
    my ($node, $from, $to) = @_;
    return () unless $node;
    return range_query ($node -> left,  $from, $to) if $to   < $node -
+> key;
    return range_query ($node -> right, $from, $to) if $from > $node -
+> key;
    return range_query ($node -> left,  $from, $to), $node -> key,
           range_query ($node -> right, $from, $to)
}
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Abigail