Update:
Roger pointed out some gross errors. Sorry! Fixed now!

Well, other than a few errors, you're on your way. I'll do a line by line on this baby and see what I think. 

my $file = "index.html";

my @codes = ("<P>", "<BR>", "UL"); # and many more

[download]
Do you mean to be inconsistent here? Or should "UL" be "<UL>"? 
open(FILE, "> $file") or die "Oops: $!";
while(<FILE>);

[download]
Your open is clobbering index.html. Not what you want. Also, you never write out to the file to actually change it in memory.
Try  while (<FILE>) { :) 
if ($_ eq @words) {
$_ =~ s/@words/lc @word/g;
}

[download]
Here's where you have your real problems. The if statement compares the file line to the number of items in @codes. Probably not what you want. Also, eq won't work unless you have only one word per line. Also probably not what you want. 
close(FILE);

[download]
Way to go.

I would rewrite it like this:
#!/usr/bin/perl -i
use warnings;
use strict;


my @codes = ("<P>", "<BR>", "<UL>"); # and many more


while(<>);
for my $word (@words) {
$_ =~ s/$word/lc $word/g;
}
close(FILE);

[download]
Call it with the filename as argument.
Of course, this is still horribly simple minded because it breaks on constructs like <P class="hi">.

Good luck fixing that with a simple regex.



Code is (almost) always untested.
http://www.justicepoetic.net/

In reply to Re: substuting a whole file by jweed
in thread substuting a whole file by Anonymous Monk

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.