How to use fork() in Perl?

In addition to your trouble using fork, you also seem to have trouble partitioning your @files array into smaller sets of four files. Maybe something like this:

for (my $i = 0; $i < @files; $i += 4) {

    # take a slice of 4 elements from @files
    my @files4 = @files[$i .. $i + 3];

    # do something with them in a child process
    if (fork() == 0) {
        ... do something with @files4 ...
        exit;   # <--- this is very important
    }
}

# wait for the child processes to finish
wait for 0 .. @files/4;

Use Parallel::ForkManager

use Parallel::ForkManager qw( );

my $pm = Parallel::ForkManager->new(int(@files/4));
for my $file (@files) {
   my $pid = $pm->start and next;

   ... do something with $file ...

   $pm->finish; # Terminates the child process
}

Note that this still creates 100 processes, it simply limits it to 25 concurrent.

If you truly want only 25 processes, you can use the following:

use List::Util            qw( min );
use Parallel::ForkManager qw( );

my $pm = Parallel::ForkManager->new(0+@files);
while (@files) {
   my @batch = @files[0..min(4, $#files)];
   my $pid = $pm->start and next;

   for my $file (@batch) {
      ... do something with $file ...
   }

   $pm->finish; # Terminates the child process
}