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.
}