I suspect you are attempting to use perl as if it was a shell. UNIX shells inspect commands and decide if they are keywords, aliases, built-ins, functions or external programs (or other exotics). This is quite unusual for a programming language, but convienient for a command-line driven shell. Perl, like most programming languages, does not work in this way.

See system, which is based upon the function of the same name in C (although it is smarter). You can also use back-ticks `` as you would in a shell, or qx.