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;