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;