I think that GrandFather's regular expression solution is the best for you but you ask what other solutions we have. davido has come up with an improved substr solution. Here's a way with split, splice and join.

use strict;
use warnings;

my $str = q{12345678};
print qq{$str\n};

$str = revByGroup($str);
print qq{$str\n};

sub revByGroup
{
    my $str = shift;
    my @chars = split m{}, $str;
    for (my $idx = 0; $idx < $#chars; $idx += 2)
    {
        splice @chars, $idx, 0, splice @chars, $idx + 1, 1;
    }
    return join q{}, @chars;
}
[download]

and the output is

12345678
21436587
[download]

It copes with an odd numer of characters, leaving the last odd character at the end of the string. I hope this is of interest.

Cheers,

JohnGG