many thanks,
the access to the registry is a little bit obscure but it seems work..
It works with system dsn too...
here my resulting code:

use Win32::TieRegistry ( Delimiter=>"/" );

$dsn="testDSN";
$odbc= $Registry->{"HKEY_LOCAL_MACHINE/Software/ODBC/ODBC.INI/$dsn/"}or  die "$^E\n";

print $odbc->{DBQ}or die $!;

I hope some day I can help someone in the same way you helped me.
ciao lorenzo*