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;
}

##</code><code>##

12345678
21436587