sub swap {
    my ($str) = @_;
    die 'No input?' unless defined $str;

    ## 0 to half-length minus one
    for my $i (0 .. (length( $str ) >> 1) - 1) {
        my $j = $i << 1;
        substr( $str, $j, 2, reverse substr( $str, $j, 2 ) );
    }
    return $str;
}