At the top of your code put:

use strict;
use warnings;
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Test your code (without actually running it) with:

perl -c test.pl
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That should highlight this and other problems.

If you are having problems understanding the error messages, you can get more detailed output by also adding this to the top of your code:

use diagnostics;
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Your current problem relates to $dbh (in $dbh->prepare) being undefined. What you did define was $sql_dbh. You've also used $dbh elsewhere in your script. Choose which one you want to use and make appropriate changes.