Ah, like this?

use threads;
use threads::shared;

my @file_list :shared = @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";
}

for (1..$num_workers) {
   async {
      for (;;) {
          my $file = do {
              lock @file_list;
              return if !@file_list;
              shift(@file_list)
          };
          process($file);
      }
   }
}

$_->join() for threads::list();
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This will work as long as you never add anything to the file list once the threads have started. (If you do, you might have worker threads exiting prematurely.)

Thread::Queue does the same, but hides a couple of the details.