Just add "warn" to your connect_mysql function and to the test code right after the call to connect_mysql. Ie.

sub connect_mysql{
....
my $dbh = DBI->connect..
warn("Got dbh '$dbh'\n");
return $dbh;
}
......
my $sql_dbh = ModTest->connect_mysql();
warn("Dbh is '$dbh'\n");

And then see what is really returned to you in both places.