This isn't too "cool", but it helped to save my sanity during my last semester of mainframe programming (blech) and it was my first "online application" written in Perl. COBOL and CICS (a character-based user interface to COBOL) use a lot of wordy syntax to represent report files and character displays. I wanted a way to input a formatted text file that looked like the report output or screen interface into a Perl script that would return the bloated COBOL WORKING-STORAGE section or CICS BMS macro statements, ready to cut and paste into my programs. So I wrote it. 8^) It's not 100% accurate with the code, but it's better than handcoding it all. It even returns a "time saved" value that estimates how much time you saved not having to write all of the wordy code.
If you want to see the programs in action, check them out at: