while( @data ) {
    /^HEADING/ ... !/^HEADING/ and push @headings, $_;
    /^TITLE/   ... !/^TITLE/   and push @titles,   $_;
    /^COMPND/  ... !/^COMPND/  and push @cmpnds,   $_;
}

##</code><code>##

$re_types = qr/(HEADING|TITLE|COMPND)/;
while (@data) {
    /^$re_types(.*)$/ ... !/^$1/ and push @{$type{$1}}, $2;
}

##</code><code>##

#! perl -slw
use strict;

my @data = <DATA>;

my (@headers, @titles, @compnds);

for (@data) {
	(/^HEADER (?:\d+ )?(.*?)$/ and push(@headers, '' ))
		...
	!/^HEADER (?:\d+ )?(.*?)$/
		and $1
		and $headers[-1] .= $1, next;

	(/^TITLE (?:\d+ )?(.*?)$/ and push(@titles, '' ))
		...
	!/^TITLE (?:\d+ )?(.*?)$/
		and $1
		and $titles[-1] .= $1, next;

	(/^COMPND (?:\d+ )?(.*?)$/ and push(@compnds, '' ))
		...
	!/^COMPND (?:\d+ )?(.*?)$/
		and $1
		and $compnds[-1] .= $1;
}
print for @headers;
print for @titles;
print for @compnds;


__DATA__
HEADER  Header 1 stuff
TITLE   Title 1 stuff
TITLE   2 more title 1 stuff
COMPND  complicated stuff 1
COMPND  2 continued complicated stuff 1
HEADER  Header 2 stuff
TITLE   Title 2 stuff
TITLE   2 more title 2 stuff
COMPND  complicated stuff 2
COMPND  2 continued complicated stuff 2
HEADER  Header 3 stuff
TITLE   Title 3 stuff
TITLE   2 more title 3 stuff
COMPND  complicated stuff 3
COMPND  2 continued complicated stuff 3
COMPND  3 continued complicated stuff 3
HEADER  Header 4 stuff
TITLE   Title 4 stuff
TITLE   2 more title 4 stuff
COMPND  complicated stuff 4
COMPND  2 continued complicated stuff 4
HEADER  Header 5 stuff
TITLE   Title 5 stuff
TITLE   2 more title 5 stuff
COMPND  complicated stuff 5
COMPND  2 continued complicated stuff 5
HEADER  Header 6 stuff
TITLE   Title 6 stuff
TITLE   2 more title 6 stuff
COMPND  complicated stuff 6
COMPND  2 continued complicated stuff 6

##</code><code>##

C:\test>221570
 Header 1 stuff
 Header 2 stuff
 Header 3 stuff
 Header 4 stuff
 Header 5 stuff
 Header 6 stuff
  Title 1 stuff  2 more title 1 stuff
  Title 2 stuff  2 more title 2 stuff
  Title 3 stuff  2 more title 3 stuff
  Title 4 stuff  2 more title 4 stuff
  Title 5 stuff  2 more title 5 stuff
  Title 6 stuff  2 more title 6 stuff
complicated stuff 1 2 continued complicated stuff 1 complicated stuff 2 2 continued complicated stuff 2 complicated stuff 3 2 continued complicated stuff 3 3 continued complicated stuff 3 
complicated stuff 4 2 continued complicated stuff 4 complicated stuff 5 2 continued complicated stuff 5 complicated stuff 6 2 continued complicated stuff 6
C:\test>