rem -- somewhere inside the big batch file --
perl -e "exit 42"
if errorlevel 43 goto noperl
if errorlevel 42 goto badperl
goto noperl
:badperl
echo Sorry, there is a foreign perl installed. Can't install.
goto fail
:noperl
rem ...
rem ... much code omitted, explained in the next step
rem ...
:fail
pause
:end