exists can only do case sensitive comparisons. Instead of doing exists, extract the keys from both hashes using keys, and then lower case them both before comparison:

Note: this solution preserves the case in both hashes by storing keys in a temporary third hash.

# make temporary hash using lower case keys only
# hash allows for fast lookup of matching key later on in
# our for loop
my %hashLowercaseKeys1 = ( map { lc($_) => 1 } keys %hash1 );

foreach my $k (keys %hash2) {
  if (exists $hashLowercaseKeys1{lc($k)}) {
    print "$k:  matched\n";
  } else {
    print "$k:  no match\n";
  }
}
[download]