Is this a script in a language other than perl

It's a Windows shell (cmd) command. If you need it in a batch file, you need to escape the %. Change %q to %%q.

I'm not familiar with cygwin

Cygwin is a OS emulator, not a shell or a scripting language. When people say Cygwin, they usually refer to bash and/or GNU tools. samtregar showed that a solution can be obtained using bash and GNU tools. I showed that such tools weren't required.

is there some significance to the use of "perl -pi.bak"

It invokes perl. -p and -i are documented in perlrun.