use strict;
use warnings;

my @TEST_STRINGS
    = ( q(), qw(
                1 12 123 1234 12345
                )
        );
for my $t (@TEST_STRINGS) {
    printf qq(%-7s => "%s"\n), qq("$t"), swap( $t );
}
exit( 0 );

##</code><code>##

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;
}

##</code><code>##

""      => ""
"1"     => "1"
"12"    => "21"
"123"   => "213"
"1234"  => "2143"
"12345" => "21435"