Gimp-Perl scripts are technically plug-ins, which are written in Perl.
Inside such a plug-in one can do everything that can be done in Perl - e.g: opening files, sockets, fork()ing and executing other programs and GUI code.
Gimp-Perl is not available for Win32 at the time of this writing, and also has a more picky installation process.
Gimp-Perl has an extension called Gimp::Pixel that enables to write plug-ins using the Perl Data Language (=PDL). This is, in fact, the GIMP's answer to the Image Toolbox of Matlab.