While some people might say "use CGI.pm or die"...
I will just say

export REQUEST_METHOD "POST"
or
setenv
or
set REQUEST_METHOD = "POST"
and then
~> perl -wT mail.cgi what=makemail otherparam=this
[download]

Also, for debugging, You can use -c to parse the script (without running it) to check for errors.
~> perl -wT -c mail.cgi what=makemail otherparam=this