Don't know if this will help you or not, but afaik, a stored proc is called with an exec statement. I would do it like so --

$action = $dbh->prepare("EXEC IBW_WebAuth $reg_num,$passwd") ;
$action->execute;
$rows = $action->rows ;
$action->finish;
print "rows is $rows\n";
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Other than that, others' notes above about -1 might also be helpful.