The core transaction processing system has 1.3 million lines of HP3000 COBOL that is pre-compiled to 13 million lines of Micro Focus COBOL, so that code can be compiled to run on UNIX hardware (using a mainframe emulator framework).
So although I'm not a COBOL dev myself, I deal with the issue you describe constantly. Want to give an inventory item an identifier that's longer than 36 characters? Too bad! That field in the legacy system is PIC X(36) so it will get truncated to that length if it's longer (and of course, padded with spaces if it's shorter, so there are lots of String.trim() calls in our Java code).