require mydir::mymod123;
$result = mydir::mymod123::mysub();
##</code><code>##

package mydir::mymod123;
sub mysub
{
  ...etc...
  return $x;
}
1;

##</code><code>##

    $modno = 123;   # Just for simplicity.  Actually this number will come from a database.
    require "mydir/mymod$modno.pm";