Anonymous Monk,
I, like the others, had a hard time understanding what exactly what you wanted. I made some assumptions:

• Product: is a delimiter
• Work needs to be done on lines between delimiters
• The first word (defined by whitespace) of each line is important and will be smaller than 64K
• The longest substring that each word ends with is what is desired
As CountZero points out, this is likely flawed

#!/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
[download]

---

---

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! —

• How do I compose an effective node title?
• How do I post a question effectively?
• Markup in the Monastery

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:
	&		&
	<		&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.