If the stuff inside the curlies looks like a word it's automaticaly quoted.

Thus $hash{undef} and $hash{'undef'} are equivalent.Try :

$ perl -MO=Deparse -e "print $hash{undef}"
[download]

If you want to use an undef as a hash key you have to use either $hash{+undef} or something like my $NULL = undef; $hash{$NULL}

Update: Erm ... hash keys are strings. Always. So $hash{+undef} us equivalent to $hash{''}. Oh well.

  Jenda