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

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

use strict;
use warnings;

#!Perl -l

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

__END__

#!Perl -lp

...
$_=dir_size $_;

__END__

dir1.txt, dir2.txt, dir3.txt

print dir_size $_ for split /, /;