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

my $emailNum;

$/ = ''; # Set readline to "Paragraph mode"

while (<DATA>) {
    if (!$emailNum || /^From:/im) {
        ++$emailNum;

        print "---- Email $emailNum\n";
    }

    print;
}

__DATA__
From: here
To: there
Data:
I have a number of e-mail messages (envelope, headers, body, sometimes including
base64-encoded attachments) which are spread out among an arbitrary number of
text files. In most cases, there are multiple messages in each text file.

I want to subset each large file so that each e-mail is saved to its own file.
So, if the source file is "mails.txt" containing three messages, I want to
create (e.g.) mails_000001.txt, mails_000002.txt, and mails_000003.txt.

From: somewhere
To: elsewhere
Data:
A second email

##</code><code>##

---- Email 1
From: here
To: there
Data:
I have a number of e-mail messages (envelope, headers, body, sometimes including
base64-encoded attachments) which are spread out among an arbitrary number of
text files. In most cases, there are multiple messages in each text file.

I want to subset each large file so that each e-mail is saved to its own file.
So, if the source file is "mails.txt" containing three messages, I want to
create (e.g.) mails_000001.txt, mails_000002.txt, and mails_000003.txt.

---- Email 2
From: somewhere
To: elsewhere
Data:
A second email