How would that work? A folded header line does contain one single \n, and then the next line begins with any whitespace. A newline (\n) followed by a non-whitespace is the beginning of a new header.

The whitespace a folded line begins with, may not neccessarly be a \n. In face \n would not be permitted, because a double \n (\n\n) would mark the start of body

Example of a non-folded header line, combined with a folded header line combined with a non-folded one:

Subject: Hi you are beutiful
Content-Type: text/plain;
    format=flowed;
    charset="iso-8859-1";
    reply-type=original
Return-Path: <example@example.org>
[download]

$fixedheaders[0] = "Subject: Hi you are beutiful";
$fixedheaders[1] = "Content-Type: text/plain;\tformat=flowed;\tcharset
+=\"iso-8859-1\";\treply-type=original";
$fixedheaders[2] = "Return-Path: <example@example.org>";
[download]

sebastiannielsen2:

In that case, I'd use:

@fixedheaders = split /\n\b/, $fixedmsgheader;

Update: checked my work with a test program:

 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>
[download]

...roboticus

When your only tool is a hammer, all problems look like your thumb.

OOps the result was a Little bit wrong. It should be:

$fixedheaders[0] = "Subject: Hi you are beutiful";
$fixedheaders[1] = "Content-Type: text/plain;\n\tformat=flowed;\n\tcha
+rset
+=\"iso-8859-1\";\n\treply-type=original";
$fixedheaders[2] = "Return-Path: <example@example.org>";
[download]

Eg with newlines Before the tabs.