but still do not understand how to fork a process and use my variables from my main process there aswell

The point is, you can't (directly). At the time of forking, all program data (including variables with their current values) are being duplicated, but from that point onwards, updates to the two copies of the variables do happen independently.  It's for that reason that you'd then need IPC.

With threads and threads::shared, OTOH, you can tell Perl which variables you want to share across the different threads.  But threads may be problematic in other ways, for example in that code (which you've loaded before creating a new thread) needs to be thread-safe — which in particular external libraries (as often needed with XS modules) sometimes aren't.

luckily, we also have forks::shared !

True, and it's probably always good to post a reminder of forks in discussions about threads. However, it should be noted that the forks module explicitly doesn't work on Windows. Follow the link to forks and do a text search for Windows for an explanation. This is particularly important since the OP is using Windows.

Perl threads are a big and rather cludgy hammer, and should only be used when you need to share lots of data between many processes. They're great to have when you truly need them, but in your case, you don't truly need them. :)

Since you know exactly how long you want to wait, you can simply use the alarm function. You've got high precision for your timing, so you should probably use the Time::HiRes module, which is almost certainly a core module for you. To get all of this to work, a skeleton of your code might look like this:

use Time::HiRes qw(alarm);

# Pre-declare any variables you'll need in play_func and in
# the main code
my %all_files;

sub play_func {
   ## Most of your originally posted code goes here, but
   ## replace the call to sleep with this:
   alarm ($song_length);
}

# You'll need to associate the alarm signal handler with
# play_func like so:
$SIG{ALRM} = \&play_func;

# Initialize your data here, such as your playlist

# Kick-off the play loop by calling play_func once:
play_func();

# Put your own looping/interface code here:
while(1) {
   # This will loop until you call 'last;' or exit.
   # When the alarm signal goes off, Perl will jump to
   # your alarm-handling code and execute that, then
   # return to whatever it was doing in here.
   # This will lead to a slight interrupt in this loop,
   # but it probably won't be noticeable.
}
[download]

Hope that's a useful alternative approach!