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

##</code><code>##

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