#!/usr/bin/perl -w
use strict;
local $/;
open(INPUT, "input.html") || die("$!");
open(OUTPUT, ">output.html") || die ("$!");

my $txt = <INPUT>;
$txt=~s/<([^> ]*)([^>]*>)/"<".uc($1)."$2"/egsi; #Updated: only element names except attributes
print OUTPUT $txt;