This is a very good question, and has been raised on p5p in the past. As far as I know (which might not be far enough - I'm doing more category theory than programming these days) there is still no simple way to do what you want.

The most concerted attempt to change this was made by Jeff Friedl in 2001. Take a look at the threads starting here, here and here. As you can see, the necessary code has all been written, but doesn't seem to have been incorporated into perl. You could try asking p5p about the status of this.

Update: On closer inspection, it seems that this has been incorporated into Perl 5.8. Look at the module Hash::Util. It seems that, in Perl 5.8, the SvREADONLY flag does prevent new keys from being added to a hash. What version of Perl are you using?

 .robin.