#!/usr/bin/perl
use strict;
use warnings;
local $/ = "\nProduct:\n";
while ( <DATA> ) {
    my @line = map { /Product:/ ? () : (split " ")[0] } split /\n/;
    print join ' + ', @line;
    print " = ", common( \@line ), "\n";
}
sub common {
    my $line = shift;
    my $short = 65536;
    for ( @$line ) { $short = length $_ if length $_ < $short };
    my $index;
    for ( --$index ; $short-- ; --$index ) {
        my $str = substr($line->[0], $index);
        for ( @$line ) {
            return substr($_, ++$index) if substr($_, $index) ne $str;
        }
    }
}
__DATA__
Product:
redball This is for Mike.
greenball This is for Dave.
Product:
smallbox This is for apples
bigbox This is for orange