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;
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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.

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