in reply to How do I modify the KEYS in a hash (copy)

Here is another way to modify the keys. 

my %Orig=('a b'=>44,  'c d'=>22, rr=>99);

my %Copy = map { $_ => $_=~ s/ /_/ } keys %Orig;
print qq(C: $_ => $Copy{$_}\n) for keys %Copy;

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Originally posted as a Categorized Answer.

In Section Seekers of Perl Wisdom