Doubleclick and @ARGV

A question for Windows experts.

I have installed a wxPerl application and associated it with files with
extension '.cho'.

When I doubleclick in the Explorer on such a file, my program is started
and gets the name of the file passed via @ARGV. Well, more or less...

I have a folder (on an NTFS filesystem) with three files:

asciiname.cho
namewith=C3=A1ccent.cho
namewith=E2=99=A1unicode.cho

=46rom within the program, I can open the file if I dclick the first and the
second name, but not the third name.

What **exactly** is passed in @ARGV when I dclick these files?
Since the filesystem is NTFS, I would expect a filename in UTF-16LE but
when I dump the content of @ARGV (using charnames to avoid encoding issues)
I get

"asciiname.cho" -- apparently ASCII

"namewith\N{LATIN SMALL LETTER  WITH ACUTE}ccent.cho" -- apparently UTF8

"namewith?unicode.cho" -- ????

Anyone have an idea what I'm doing wrong?