One solution is going through all the keys of one the hashes and check if those are defined on the second one.

foreach my $key (keys %csahash) {
    if(defined $cathhash{$key}) {
        print "$key matches. It's values are $csahash{$key} and $cathh
+ash{$key}\n";
    }
}
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update
having written a foreach, thought it would be a good candidate for a 'map' solution.

map { print "$_ matches. It's values are $csahash{$_} and $cathhash{$_
+}\n" if(defined $cathhash{$_}) } keys %csahash;
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update 2
updated the map solution by adding, the mistakingly droped, 'defined' to the if clause.
Thanks johngg for pointing that out.