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.

In reply to Re: Transposition of each letter pair in string by Aristotle
in thread Transposition of each letter pair in string by Cody Pendant

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.