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

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

##</code><code>##

for my $key ( keys %doublet ) {
    if ( exists( $triplet{$key} )) {  # need to check values
        next if ( $doublet{$key}[0] eq $triplet{$key}[2] and
                  $doublet{$key}[1] eq $triplet{$key}[1] );
    }
    # get here if $key does not exist in %triplet,
    #       OR if $triplet{$key} is not equivalent to $doublet{$key}
    print "$key\n";
}