dir.txt = [ dir1.txt, dir2.txt, dir3.txt]

use Win32::DirSize;
open (FILE, "dir.txt") || die "can not open file\n";

##</code><code>##

use strict;
use warnings;

##</code><code>##

#!Perl -l

use strict;
use warnings;
use Win32::DirSize;

open my $fh, '<', 'dir.txt' or 
  die "Can't open `dir.txt': $!\n";

chomp, print dir_size $_ while <$fh>;

__END__

##</code><code>##

#!Perl -lp

use strict;
use warnings;
use Win32::DirSize;

BEGIN {@ARGV='dir.txt'}
$_=dir_size $_;

__END__

##</code><code>##

dir1.txt, dir2.txt, dir3.txt

##</code><code>##

print dir_size $_ for split /, /;