What OS are you on? If you're on win9x with command instead of cmd as the shell, I don't think Inline::C will work. I tried a number of things (including installing a win9x version of cmd) and also asked Ingy (author of Inline::C) and he didn't know of any solutions either.
speaking from total ignoarance.. couldn't Inline::C just check $ENV{'COMSPEC'} if $^O was a win32 ? or is it something in the different of cmd and command shells? the cmd shell is quite a bit better.