Something like this?

#!/usr/bin/perl

# http://perlmonks.org/?node_id=1125305

use strict;

$_ = join '', <DATA>;

print "$1 to $3 := ", $4 - $2, "\n" while /^(.*) (.*)\n(?:\1 .*\n)*(?=
+(.*) (.*))/gm;

__DATA__
a 123
a 125
b 127
a 129
a 130
b 131
a 132
b 133
[download]

Produces:

a to b := 4
b to a := 2
a to b := 2
b to a := 1
a to b := 1
[download]