I was picturing it like this (incorrectly thinking substring would modify $_), where I think the order does matter, and I can't predict what would happen:

    $hash{ do{ $_ =~ s/(..)$//; $_ } } = delete $hash{ $_ } for keys %
+hash;
[download]

What appears to happen is that the lvalue is evaluated first, so I end up deleting the modified key into the new key position (leaving all the old keys in place and defining new keys to undef); but I couldn't have guessed until I tried it, so I'd tend to lean toward a for() loop like my first post.

And I think that was jwkrahn's point too:

    $hash{ substr $_, 1, 20, "" } = delete $hash{ $_ } for keys %hash;
[download]