Okay, I think I've solved it. I just discovered Recipe 6.8 in the Perl Cookbook 2nd Edition (p. 199), which uses the .. and ... operators to extract a range of lines. So long as I know that the first header line will always have 'H' as the first character, and as long as I know what the last header line might look like, then I should have no problem. I've never seen them use anything other than '<' or '[' on that last line.
Thanks for your help. I was actually looking through the cookbook for another problem, and this one just popped out. Go figure, eh?