The following uses HTML::Tokeparser and should give you a starting point:

use strict;
use warnings;

use HTML::TokeParser;

my $p = HTML::TokeParser->new( "file.html" );

while ( my $t = $p->get_token )
{
    #forward comments, text and declarations
    if ( $t->[0] =~ /[CDT]/ )
    {
        print $t->[1];
    }
    
    #uppercase start tags
    elsif ( $t->[0] =~ /S/ )
    {
        print
          "<",
          uc($t->[1]),
          " ",
          join (" ", map { uc($_) . '="' . $t->[2]->{$_} . '"' } @{$t-
+>[3]}),
          ">";
    }
    
    #uppercase end tag
    elsif ( $t->[0] =~ /E/ )
    {
        print uc($t->[2]);
    }
   
    #forward processing instruction
    elsif ( $t->[0] =~ /PI/ )
    {
        print $t->[2];
    }
}
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