Some time ago, Yary and I exchanged emails for walking directories in parallel. The following is our collaborative effort modified for the OP task.

use strict;
use warnings;

# usage: ./collect_files.pl <user> /var/tmp /tmp
# output is saved under /var/tmp/test_<user>.file.list

package Number;

sub new {
   my ($class, $self) = (shift, shift || 0);
   bless \$self, $class;
}

sub decr { lock $_[0]; --${ $_[0] } }
sub incr { lock $_[0]; ++${ $_[0] } }
sub get  { lock $_[0];   ${ $_[0] } }

package Main;

use threads;
use threads::shared;
use Thread::Queue;

use File::Spec::Functions qw(catfile);
use MCE;

# Get user info and directories to start with
my $user   = shift;
my $uidNum = getpwnam($user);

my @start_dirs = @ARGV
   or die "Please specify one or more starting directories, stopped";

-d or die "No such directory: $_, stopped" for @start_dirs;

# Shared queue and counter
my $shared_work = Thread::Queue->new( @start_dirs );
my $free_count  = shared_clone( Number->new(0) );

# Open output file
open my $user_file, ">", "/var/tmp/test_$user.file.list"
   or die "cannot open file: $!";

sub traverse {
   $free_count->decr;

   my ( $dev, $ino, $mode, $nlink, $uid, $gid );
   my @work = $_;

   while ( $_ = shift @work, defined ) {
      my ( $dir, $path, @paths ) = ( $_ );
      opendir DH, $dir or next;

      for ( readdir DH ) {
         next if $_ eq '.' || $_ eq '..';
         if ( -d ( $path = catfile($dir,$_) ) ) {
            ( @work < 15 )
               ? push @work, $path
               : $shared_work->enqueue($path);
            next;
         }
         push @paths, $path;
      }

      for my $file ( @paths ) {
         if ( -f $file ) {
            ( $dev, $ino, $mode, $nlink, $uid, $gid ) = lstat( $file )
+;
            MCE->print( $user_file, "$file:$uidNum:$gid:$mode\n" )
               if $uid == $uidNum;
         }
      }
   }

   # Done with our work, let everyone know we're free
   $shared_work->enqueue( (undef) x MCE->max_workers )
      if $free_count->incr == MCE->max_workers && !$shared_work->pendi
+ng;
}

my $mce = MCE->new(
   max_workers => 8,
   user_begin => sub { $free_count->incr },
   user_func => sub { traverse() while ($_ = $shared_work->dequeue, de
+fined) },
);

$mce->run;

close $user_file;
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Kind regards, Mario