#! perl -slw
use strict;
use threads;

sub worker {
    my( $dir ) = @_;
    my $total = 0;
    opendir my $dh, $dir or die "$dir : $!";
    while( my $file = readdir $dh ) {
        $total += -s "$dir/$file";
    }
    closedir $dh;
    return $total;
}

my @threads;

for my $dir ( @ARGV ) {
    if( -d $dir ) {
        push @threads, threads->new( \&worker, $dir );
    }
}

my $total;
$total += $_->join for @threads;

print "Total bytes in files in directories [@ARGV] is: ", $total;
__END__
C:\test>612156 3 4 5 data
Total bytes in files in directories [3 4 5 data] is: 654203874