pp --clean does not seem to work ... no proof ...

You're confused, --clean works

when pp is creating an executable , it putts stuff into temp, this is what you're confusing for --clean not doing its job

$ rm -rf goner-temp

$ md goner-temp

$ set TEMP=goner-temp

$ set TMP=goner-temp

$ findrule goner-temp |wc --lines
1

$ pp --clean coy.pl

$ findrule goner-temp |wc --lines
109
[download]

Running the executable doesn't increase the number of files in temp, this means --clean is working

$ a.exe 1>NUL 2>NUL

$ findrule goner-temp |wc --lines
109
[download]

Emptying temp, and running the executable shows no files in temp

$ rm -rf goner-temp

$ md goner-temp

$ findrule goner-temp |wc --lines
1

$ a.exe 1>NUL 2>NUL

$ findrule goner-temp |wc --lines
2
[download]

Without --clean option the number of files doubles after running the executable

$ rm -rf goner-temp

$ md goner-temp

$ pp coy.pl

$ findrule goner-temp |wc --lines
109

$ a.exe 1>Nul 2>NUL

$ findrule goner-temp |wc --lines
220
[download]

--clean worked as designed, its working

Could it be that it works on windows and is broken on linux? Not likely -- now you know how to check