#! /usr/local/bin/perl

$string1 = "the date is today";
$string2 = "the date is tomorrow";

$concat = $string1 . "#" . $string2;

if ( $concat =~ m/^(.*)(.*)#\1(.*)$/ ) {
    print "matching part: $1\n";
    print "difference: $2\ndifference: $3\n";
} else {
    print "strings $string1 and $string2 do not match!\n";
}

##</code><code>##

matching part: the date is to
difference: day
difference: morrow

##</code><code>##

...
$concat = $string1 . "#!!#" . $string2;

if ( $concat =~ m/^(.*)(.*)#!!#\1(.*)$/ ) {
...