cat splitest.pl
#!/usr/bin/env perl
use strict;
use warnings;

my $t = q{
Subject: Hi there
Content-Type: text/plain;
    format=flowed;
    charset="iso-1189-1";
Return-Path: <example@example.org>
};

my @fields = split /\n\b/, $t;
print join("\n***\n", @fields), "\n";

localadmins-MacBook-Pro-2:~ [mmason]
$ perl splitest.pl

***
Subject: Hi there
***
Content-Type: text/plain;
    format=flowed;
    charset="iso-1189-1";
***
Return-Path: <example@example.org>