I have a question about filehandles, even after reading the docs... I
know how to start a subprocess from perl and read from it. But how do
I write to the parent process that started me, the perl script?

I have tcl script from somewhere that calls a perl script I'm writing.
The tcl script says:
     set channel [open |./randomize RDONLY]
It calls the perl script "randomize" which reads from a file, does
things to the content and should make it available for reading from
the tcl script. But how do I do that? Somehow I keep on getting
messages that the input is empty when I run the tcl script.

This would be what "randomize" looks like:
open INPUT, "inputfile";
while (<INPUT>) {
     ...
     print $result;
     # I've tried all sorts of things here with | < > at the beginning
or the end,
     # exec and backticks...