use Win32::Clipboard;

my @arr = Win32::Clipboard::EnumFormats();
print "@arr\n";

foreach my $format (@arr) {
    print "Format: $format\n";
    print Win32::Clipboard::GetAs($format);
    print "\n\n";
}
[download]

This shows all available formats and their values. Looks like CF_HTML is 49485 and therefore to get the HTML version of the text in clipgoard you'd use Win32::Clipboard::GetAs(49485).

Not sure why isn't the HTML included in the list of exported constants.