I'm surprised noone mentioned reverse. Here's a slightly insidious solution.

use strict;
use warnings;
use Test::More q(no_plan);

my $string = "thisisatest";
for my $i (0 .. (length($string) - 2)) {
        my @t = ($string)x2;

        # original
        substr($t[0], $i, 1) = substr($string, $i + 1, 1);
        substr($t[0], $i + 1, 1) = substr($string, $i, 1);

        # mine
        $_ = reverse $_ for substr($t[1], $i, 2);

        is($t[0], $t[1], "$t[0] eq $t[1]");
}

__END__
ok 1 - htisisatest eq htisisatest
ok 2 - tihsisatest eq tihsisatest
ok 3 - thsiisatest eq thsiisatest
ok 4 - thiissatest eq thiissatest
ok 5 - thissiatest eq thissiatest
ok 6 - thisiastest eq thisiastest
ok 7 - thisistaest eq thisistaest
ok 8 - thisisaetst eq thisisaetst
ok 9 - thisisatset eq thisisatset
ok 10 - thisisatets eq thisisatets
1..10
[download]

It relies on the fact that for aliases rather than copies its variable - and substr returns an Lvalue you can assign to, so that snippet is the equivalent of the more verbose substr($t[1],$i,2) = reverse substr($t[1],$i,2);.

That is, in my opinion, the most Perlish solution.

Makeshifts last the longest.