As already said, the third paramenter to substr is the length of the addressed substring.

But for the sake of diversity, here a regex that would work:

# ...
for my $line (@lines) {
   print $line;
   $line =~ s/^(.{11})DC/\1  /;
   print MYOUTPUTFILE $line;
}
# ...
[download]

And if your script is just for filtering text files you could even reduce the whole thing to a simple command line (if you have a shell, that is):

   elmex@home: ~# cat file.txt.mrk | perl -ne 's/^(.{11})DC/\1  /; pri
+nt' >> test.tmp
[download]