my $gpg = '/home/gpghrp/.gnupg';

my $file = "$gpg/scripts/gpg_encr.saved";
my $temp = "$file.new";

open(OLD, '<', $file)
   or die "Can't open $file: $!\n";

open(NEW, '>', $temp)
   or die "Can't create $temp: $!\n";
      
while (<OLD>) {
   # Faster than regexp for plain text.
   if (index($_, '(?)readonly tlx=') >= 0) {
      substr($_, 13, 8, "blah");
   }

   print NEW $_;
}

close(NEW);
close(OLD);

rename($temp, $file)
   or die "Can't update $file: $!\n";
[download]

or as a one-liner:

perl -i -pe '/\Q(?)readonly tlx=\E/ && substr($_,13,8,"blah");' /home/
+gpghrp/.gnupg/scripts/gpg_encr.saved
[download]