I've got the same problem as http://forums.movabletype.org/2009/12/wide-character-in-subroutine-entry-error.html described.
My old site was built with version 4.3, and I update to version 5.
This site has lots of Japanese and Chinese characters.
When I try to publish the blog, I got the "wide character in subroutine entry" error.
The problem is in cgi-bin/mt/plugins/Markdown/Markdown.pl.
Here is the solution:
add one line in head:
use Encode qw(encode_utf8);
modify line 286:
my $key = md5_hex($1);
my $key = md5_hex(encode_utf8($1));
Reported on Movable Type 5