Are you able to execute an even simpler script:

print "Hello, world!\n";
[download]

Does ActivePerl expect a "shebang" line at the top of this Perl script (#!c:\perl\perl.exe or whatever)? It sounds almost like Windows doesn't know how to execute the script. Net::SMTP has no open, system or exec calls in it, so it's odd that it would be generating any error at all like this.