#!/usr/bin/perl -w

use strict;

my $i = do { local $/; <DATA> };

my @addresses = ();

while ($i =~ /^From:\s+(.*)$/mg) {
  push @addresses, $1;
};

print join "\n", @addresses;

__DATA__
From:    21755603@ad.cd.net
Date:    Wednesday, April 11, 2001  10:08 PM


From:    21742877@rd.cd.net
Date:    Wednesday, April 11, 2001  07:13 PM

From:    Grouch
Date:    Sunday, February 18, 2001  08:08 AM

#Anything after "Grouch" does not get matched.

From:    21742877@fr.tx.net
Date:    Wednesday, April 11, 2001  07:13 PM