That's what you get for using cygwin. It works just fine from AS (or any native) perl.

The problem lies in the fact that the start command is a shell (cmd.exe) built in command, but when you use system under cygwin, it doesn't fallback to using cmd.exe if it cannot find an executable to run the command you supply.

The answer may be to do system "cmd.exe /c start calc.exe";

Other alternatives include using cygwin's fork.

BTW: Under AS/native perl, you can run a command asynchronously using system 1, "calc.exe";