So you say that it does/did not fork() at all? Have you investigated with a simple test script to see if forks happen, how many and under what conditions with what system messages? Can the OS limit the number of forks? Note that Windows does not have a native fork() and so Perl emulates that somehow. See perlfork for the caveats and warnings. (I am exclusively working with Linux so I can't help with anything else)

Yes, it didn't fork at all. I tried a test script and both, the fork and threads worked. In my program, it was the first fork that failed. I couldn't even print the return value and it doesn't print any error messages on exit.

I tested now with Strawberry 64bit and I wasn't able to get it work. But I tried again starting with a fresh 32bit version at that still does work.

Yes, I read about perlfork.

fluks,

I have 64-bit Strawberry 5.30, and it has no trouble correctly running fork:

#!perl

use strict;
use warnings;
use feature 'say';
use Config;

BEGIN {
    $| = 1; # autoflush
    say "perl $^V, uname = ", $Config{myuname};
}

END { say scalar localtime, "\tprocess $$ is exiting"; }

my $pid = fork();
if($pid==0) {
    say scalar localtime, "\tI am the child $$";
    exit;
}
say scalar localtime, "\tI am the parent $$";

__END__
perl v5.30.0, uname = Win32 strawberry-perl 5.30.0.1 #1 Thu May 23 12:
+20:46 2019 x64
Fri Jul 31 06:32:31 2020    I am the parent 10128
Fri Jul 31 06:32:31 2020    process 10128 is exiting
Fri Jul 31 06:32:31 2020    I am the child -5108
Fri Jul 31 06:32:31 2020    process -5108 is exiting
[download]

What happens if you run the same code?

you are presumably running your actual program inside a web server? Perhaps that's the one blocking the forks? If your test script forks then web server configuration could be one of the problems. There are log files for both the server and windows