I don't know about efficiency, but this is pretty confusing:

my $sth = $dbh->prepare($sql_last);
# ...
while (my $pointer = $sth->fetchrow_hashref){
    my $sql_back = '...';
    my $sth = $dbh->prepare($sql_back);
    while (my $pointer = $sth->fetchrow_hashref){
        # ...
        my $sth = $dbh_2->prepare($sql_net);
# ...
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The rules of lexical scoping make this all work, but I highly recommend that each $sth and $pointer (which would be more accurately, but no more descriptively, called $hash_reference) have their own name. If you use warnings (and I recommend that too), these things will toss out messages like "my" variable $x masks earlier declaration in same scope.

Does this actually hang, or just take a long time? Adding some print statements in your loops might help you see what it's doing. For general efficiency tuning, I recommend Profiling your code.