Perl is very wise. I have not tested, but I assume that in the case of %foo = (%foo, %bar); that the values of %bar that share keys with %foo will over write the value in the cat and the value in the new %foo will be from %bar?

Sorry that sentence was so long, I got caught up in the moment. I use this, not only to garuntee %bar will over write %foo, but so anyone who looks at the code after me will have no dougts as to what I was attempting:

foreach my $key (keys(%bar)){
   $foo{$key} = $bar{$key};
}
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It is iterative, but shouldn't consume too much memory. This will even pass pointers kept in values. I'm afraid a cat would expand the array I was pointing to aswell. I have never tried though.

-T