# X is regex stuff to detect start of "interesting region", Y detects end
if ($clob =~ /(.*)(X.*Y)(.*)/) {
   my ($stuff_before, $stuff_to_edit, $stuff_after) = ($1, $2, $3);
   $stuff_to_edit =~ s/foo/bar/g;
   $clob = $stuff_before . $stuff_to_edit . $stuff_after;
}