First of all, unless you use a real html parser, you are going to not handle all HTML correctly this way. But if you really want to do it by hand, I'd do something like this: 
use strict;
use warnings;

my $file = "index.html";
my @codes = ("<p>", "<BR>", "<UL>");
my $codes_regex = join "|", map quotemeta $_,
    sort { length $b <=> length $a } @codes;

# slurp the file
open my $in, "<", $file or die "couldn't open $file: $!";
my $text = do {local $/; <$in>};
close $in;

# lower case all the codes in text
$text =~ s/($codes_regex)/lc $1/gie;

# write the file
open my $out, ">", $file or die "couldn't open $file: $!";
print $out $text;
close $out;

[download]
The sort is only necessary if some codes are proper substrings of others (e.g. "ab" and "abc") and prevents "abc" from matching just "ab".

The quotemeta is needed if your codes have characters in them that are special to regexes.

In reply to Re: substuting a whole file by ysth
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.