You are mixing a statement handler with its result.

$subs_rst = $subs_str->execute(); # execute subnet select
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This instruction returns the execution result, i.e. if the query was executed correctly. (See the DBI docs).

For your fetch statement you need to use $subs_str, i.e.the statement handler.

As a side note, be aware that the execute method DOES NOT return the number of rows for a SELECT query.

Moreover (this is not related to your problem, but it can create other unpleasant side effects) you should not interpolate variables in your queries, but use placeholders instead.

Update
By deleting the original content, you made this answer - and the other ones below - almost meaningless. It's a pity, because the value of a PerlMonks node is in the question as much as in the answer. Other people can benefit from your mistakes. Don't be ashame of that. Everybody makes mistakes. The important thing is to learn from them, so please put the question back in place.

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It makes the thread confusing, and means that someone else reading it won't learn anything by reading it.

Michael

                while ( @row = $subs_str->fetchrow_array ) {
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Your variables are confusing. I can't tell if $subs_rst is global, and is thus the same one being used in the sub and outside of it. If so, you're clobbering the statement handle in the sub with the return value of execute.

First thing I'd suggest is to use strict so you can tell if you're blowing away something....

You need to call fetchrow_array in the while condition using the statement handle $subs_str (I think that's the one you mean), not the result of the previous execute.

            while ( @row = $subs_str->fetchrow_array ) {
                recursive_print_traversal($row[0]);
            }
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You are on very thin ice with your variables. In a recursive function it is important to get scoping right. Make your variables lexical with my or else localize them. The easy check for that is to make your code run under strict.

$addr = "select * from address_tbl where Subnet=?";
$subs = "select * from subnet_tbl where VLAN=?";
$addr_str = $dbh->prepare($addr); # prepare address select
$subs_str = $dbh->prepare($subs); # prepare subnet select

#sth is the statement handle from your original query
while(@row = $sth->fetchrow_array()) {
   $addr_str->execute($row[0]);
}
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If you want the recursion I suggest to make hte queries and the prepared statement handle global then your sub just executes.