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

my %Copy = map {my $y=$Orig{$_}; #Save Copy of Val
                tr/ /_/;
                $_ => $y}
             keys %Orig;

print qq(C: $_ => $Copy{$_}\n) for keys %Copy;
print qq(O: $_ => $Orig{$_}\n) for keys %Orig;
[download]

my %Copy;
@Copy{ map { tr/ /_/; $_ } keys %Orig } = values %Orig;
[download]

This would be efficient in the case where there were a relatively small number of keys that required modification.

Use the previous method (copy the hash) if the number of keys requiring modification is large.

my %o=(this=>33, 'th at'=>44, other=>55,'some thing'=>66,'or another'=
+>77,none=>88); 

while (my ($k,$v)=each %o){ 
 my $ModifiedK = $k;
 next unless  $ModifiedK =~s/\s/_/;
 delete $o{$k}; #This is SAFE !! RTFM.
 $o{$ModifiedK}=$v;
};

print qq(Mod: $_\t=>$o{$_} \n) for sort keys %o"
[download]

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;
[download]

Originally posted as a Categorized Answer.

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;
[download]

Originally posted as a Categorized Answer.

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;
[download]

Originally posted as a Categorized Answer.