ssh -fNg -L 6603:127.0.0.1:3306 remote_user@mysql.example.com

##</code><code>##

my $ssh_pid= open("ssh -fNg -L 6603:127.0.0.1:3306 remote_user@mysql.example.com |")
    or die;

my $dbh= DBI->connect("dbd:mysql...");

...

# tear down the connection
kill -9 => $ssh_pid;