use strict;
use warnings;
use 5.010;


use threads;
use threads::shared;

my @file_list = (
    'file1',
    'file2',
    'file3',
    'file4',
    'file5',
);


my $nofiles = @file_list;      #number of files 
my $currfile = 1;              #current number of file to process

my %MSG : shared;              #shared hash

my $thr0 = threads->new(\&process, shift(@file_list), 'thr0');
$currfile++;

my $thr1 = threads->new(\&process, shift(@file_list), 'thr1');
$currfile++;

while(1) {
 if ($thr0->is_joinable()) {
  $thr0->join;
        #check if there are files left to process
  if($currfile <= $nofiles){ 
   $thr0 = threads->new(\&process, shift(@file_list), 'thr0');
   $currfile++;
  }
 }

 if ($thr1->is_joinable()) {
  $thr1->join;
        #check if there are files left to process
  if($currfile <= $nofiles){
   $thr1 = threads->new(\&process, shift(@file_list), 'thr1');
   $currfile++;
  }
 }
}

sub process{
       my($file, $thr_name) = @_;

       print "$thr_name opening $currfile of $nofiles\n";

       #do some stuff
       sleep int(rand 5);

       print "$thr_name done with $currfile of $nofiles\n";
}

--output:--
thr0 opening 1 of 5
thr1 opening 2 of 5
thr1 done with 2 of 5
thr1 opening 3 of 5
thr1 done with 3 of 5
thr1 opening 4 of 5
thr0 done with 1 of 5
thr0 opening 5 of 5
thr1 done with 4 of 5
thr0 done with 5 of 5