Yes, there are a few problems with that code:

• 1) Your subroutine relies on the global variable $SQL, instead of a passed parameter.
• 2) Your $SQL statements are vulnerable to injection attacks since they aren't escaped and/or you're not using placeholders.
• 3) You rolling your own to browser error reporting, instead of letting CGI::Carp do the work for you.

#!/usr/bin/perl -wT

use CGI;
use CGI::Carp qw(fatalsToBrowser);
use DBI;

use strict;
use warnings;

...

    my $sth = $dbh->prepare(qq{UPDATE $regtable SET favorites=? WHERE 
+ID=?});
    $sth->execute($favorites, $ID) or die $dbh->errstr;

...
[download]

The above is all you need to accomplish the same thing. If there is an error with your statement, then CGI::Carp will trap it and display it in the browser and also the error log.