Death to Dot Star is a good read.

your regex is capturing to much to fast. This is better:

#!/usr/bin/perl -w
use strict;
use warnings;

my $default_from_domain = 'www.domain.com';
my $unsub = 'un-foo@foo.com un-foo@foo.com';
print $unsub, "\n";

$unsub =~ s/(un-[^\@]+)\@[^ ]+/$1\@$default_from_domain/g ;
print $unsub;
[download]

update:
Assuming the optimizer did not exist your regex would capture as follows (not sure how the optimizer optimizes so I am going to pretend it is not there): the (un-.*) would capture from the first place it sees un- in your string and then the .* makes it capture from this point on. It will then try to match the next thing in your regex which in your case is \@ it will thus backtrack until it finds one. At this point you have "un-foo@foo.com un-foo@" matching. The little engine that could then moves on and tries to match the next thing. Which is again .* which will of course capture the rest of the string.

So far you have the following then: $1 contains "un-foo@foo.com un-foo" and $2 contains "foo.com". Anyhow since it captures the whole string right of the bat it will not find another match.

-enlil