e.g. simply `$(G.perl) -e 'print "z" x 8192; print "ENDMARK\n" ' would do the `echo_command` IIUC.

I would actually recommend simply packaging the starting point as a file with the test as `$(this.promise_filename).txt` so you're absolutely sure there are no strange system-level interferences from a missing Perl or whatever.  Yeah, it's not nifty but it will DTRT.