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for my $key ( keys %one_hash ) {
    next if ( exists( $other_hash{$key} ));

    # now do something with this $key, which exists only in %one_hash.
+..
}

for my $key ( keys %doublet ) {
    if ( exists( $triplet{$key} )) {  # need to check values
...
    #       OR if $triplet{$key} is not equivalent to $doublet{$key}
    print "$key\n";
}