The problem is, in the discussion-space of 'computer programming languages' (especially versatile ones like perl) there *are* no rare character sequences.

That I do not believe. Sure, every character sequence can occur inside a perl program (just put it inside quotes), but many sequences are rare. And the current delimiters, [ ] are one of the most common. [[ ]] would be far less common. Sure, one can make Perl code that uses [[ ]], but if you'd do some statistics on <code> fragments on Perl monks, you'll see that [ ] is uses far, far more often than [[ ]].