That one almost works, except that I end up with an extra (empty) value in $stuff2[0],

That's because using split there will always be an implied empty field preceding the first tag. You could just shift it off the array before building your hash.

Personally, I think I'd use m/// for this:

#! perl -slw
use strict;
use Data::Dump qw[ pp ]; $Data::Dump::WIDTH = 50;

my %hash = do{ local $/; <DATA> } =~ m[(TAG\d=)\s+(.+?)(?=TAG|\Z)]gsm;

pp \%hash;

__DATA__
TAG1= data
TAG2= more data
TAG3= even more data that sometimes has = and
runs on to more
than one line
TAG4= still more
[download]

Produces:

c:\test>junk15
{
  "TAG1=" => "data\n",
  "TAG2=" => "more data\n",
  "TAG3=" => "even more data that sometimes has = and\nruns on to more
+\nthan one line\n",
  "TAG4=" => "still more",
}
[download]