Use the /s option to s/// to allow the period to match newlines as well. This works for my multiline comment test:

my $stuff = do { local $/; <DATA> };
$stuff =~ s/<!--.*?--\s*>//gs;
print "$stuff\n";

__DATA__
asdfasdkf <!--
 asdfaskdf
-->
Hello <!-- World -->
[download]

Please don't make me use the modules!

You make it sound like they're haunted or something...

Update: Hey, your second regex worked for this data. Are you sure it doesn't work for you?
Update2: The spec allows for whitespace between the closing -- and >: updated the regex accordingly.

blokhead