There's no need to create a new thread for each file.

use threads;
use threads::shared;

use Thread::Queue;

my @file_list = @ARGV;
my $num_workers = 2;

my %MSG :shared;

sub process {
   my ($file) = @_;
   print "Opening $file\n";
   if(some condition){
      lock(%MSG);
      #write stuff to hash
   }
   print "Closing $file\n";
}

my $q = Thread::Queue->new(
   @file_list,
   (undef)x$num_workers,
);

for (1..$num_workers) {
   async {
      while (defined(my $file = $q->dequeue())) {
          process($file);
      }
   }
}

$_->join() for threads::list();
[download]

Or since you never add anything to @file_list,

use threads;
use threads::shared;

use Thread::Queue;

my @file_list = @ARGV;
my $num_workers = 2;

my %MSG :shared;

sub process {
   my ($file) = @_;
   print "Opening $file\n";
   if(some condition){
      lock(%MSG);
      #write stuff to hash
   }
   print "Closing $file\n";
}

my $q = Thread::Queue->new(@file_list);
for (1..$num_workers) {
   async {
      while (defined(my $file = $q->dequeue_nb())) {
          process($file);
      }
   }
}

$_->join() for threads::list();
[download]

( Oops, way too slow at posting this )