Searching through MCE repo, I found mce_grep which ships with MCE. It depends on the OS vender including it or a separate package. The script calls an external binary, grep in this case. One can rename the file, copy, or make a symbolic link to the script itself to have it called another external binary.
It borrows bits from IPC::Run3 for capturing STDOUT/ERR on Unix platforms, but an attempt for lesser overhead it seems. See here. So this looks to be a full-fledged demonstration for doing just that; IPC::Run-like capability to run a pool of C/C++ workers. It exists :).
See also: The pure-Perl grep-like implementation is found in the mce-examples repo named egrep.pl.
Digging more, there lives another full-fledged demonstration calling Inline::C and an external C++ binary. This lives in the mce-sort repo. The Inline::C bits and corresponding user function partitions the data. Sorting the individual partitions in parallel (via an external C++ binary) is done here.