use File::Temp qw(tempfile);
my $file = 'test.txt';
my($tmpfh, $tmpfile) = tempfile();
open my $fh, '<', $file;
binmode $fh;
while (<$fh>) {
  # Process $_ here
  print $tmpfh $_;
}
close $fh;
close $tmpfh;
rename $tmpfile, $file;

##</code><code>##

use Fcntl qw(:seek);
use File::Temp qw(tempfile);
my $file = 'test.txt';
my $tmpfh = tempfile();
open my $fh, '<', $file;
binmode $fh;
while (<$fh>) {
  # Process $_ here
  print $tmpfh $_;
}
close $fh;
seek $tmpfh, 0, SEEK_SET;
open my $fh2, '>', $file;
binmode $fh2;
print $fh2 $_ while <$tmpfh>;
close $fh2;
close $tmpfh;