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