Yes, I think you're tight about making things harder than necessary... I came up with this solution. It makes for ugly spaghetti code, I guess, but it seems to work(TM). First, I need to slurp in the entire file since I want to match across newlines: 
my @lines = (<>);
my $text = join "", @lines;

[download]
Then, I test for each LABEL from LABEL O 0 to LABEL O 255. If the label exists, write it to another file, adding the new instructions. If it doesn't exist, just write the new instructions: 
if ($text =~ m/(   \(LABEL O 1\))(\n.+?)(   \(STOP\)\n)/s) {
    $labelo1 = $1;
    $labelo1instructions = $2;
print "$labelo1\n   NEWLIGINSTRUCTIONS HERE$labelo1instructions";
}
else {
print "(LABEL O 1)\n   NEWLIGINSTRUCTIONS HERE\n";
}

[download]
I tried Tie::File, but couldn't get my head around it. And everything I tried with it was incredibly slow, so I went back to the brute-force approach... Thanks for all your help!!

In reply to Re^4: adding lines at specific addresses by pindar
in thread adding lines at specific addresses by pindar

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.