Yup, thought of that :) - but then when I have problems later on (as I always do), the first things the monks would say is use -w (well, after use strict).
Looking at the documentation for the module, it says of the add method:
$obj->add( $key, $value );
Adds a new key/value pair to the hash. $key can be a Regexp or a string (which is compiled into a Regexp).
If $key is already defined, the value will be changed. If $key matches an existing key (but is not the same), a warning will be shown if warnings are enabled.
and I suspect that the final paragraph is coming into play, but I can't see why.