Well, it's not exactly correct HTML, so your best bet would probably be, to strip out the <% %> tags beforehand.

something like

#!perl
local $/;
$_ = <STDIN>;
s/<%.*?%>//sg;
s/^%.*//g;  # i think mason also interprets lines starting with % as p
+erl code.
print;
[download]

should do it.