Assumption: there is only one 'mapping' in each string.

Remove the leading characters that match, remove the trailing characters that match. What's left is the difference. If my assumption is wrong, then this won't do it for you. 
use strict;
use warnings;

binmode(DATA, ":encoding(UTF-8)");

my %normalize;

while (<DATA>) {
    my ($string, $normalized) = split;

    # convert to arrays (avoid unicode issues?)
    my @string = $string =~ m/\X/g;
    my @normalized = $normalized =~ m/\X/g;

    # skip matching the beginning chars
    while (@string and @normalized
        and $string[0] eq $normalized[0]) {
        shift @string;
        shift @normalized;
    }

    # skip matching end chars
    while (@string and @normalized
        and $string[-1] eq $normalized[-1]) {
        pop @string;
        pop @normalized;
    }
    my $key = join("", @string);
    $normalize{$key} = join("", @normalized);

    print "'$key'  => '$normalize{$key}'\n";
}

__DATA__
ABCÅD  ABCD
ABCÄD  ABCëëD
ABCááD ABCèD

[download]
Produces the results: 
'Å'  => ''
'Ä'  => 'ëë'
'áá'  => 'è'

[download]

In reply to Re: Characters in disguise by ruzam
in thread Characters in disguise by Anonymous Monk

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.