It is opaque for two reasons: 1/ because the comma operator is seldom used so little understood. I don't mean by you (or me even - I have occasionally used it in the past), but in general. (I know this because of the complaints I've received from co-workers when I've used them - "Are you allowed to do that!".) 2/ because the comma as operator is hard to see compared to how important its correct interpretation is to understanding the code. Most often commas are used in lists and tend to be treated as something the parser needs to do its job, but are essentially ignored by people reading the code - white space and layout are generally much better cues for understanding the form of the code.

Well this code is nicely indented and has huge clues as to what is important or not.

while ( (print "Enter Number: "), 
        (my $line=<STDIN>) !~ /^\s*d(?:one)?\s*$/i  
      )
{....}
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The above sort of statement is the single exception that I use the comma operator. I agree that indenting and spacing is one of the most important things that can be done when writing code.

In the above, maybe the reader or maintainer couldn't write this in the first place, but he/she will get the "gist" of the idea of what this while loop does.

Yes, the comma operator is rare. This is the only situation that I know of where it it justified. But "rare" does not mean "never".

I think my regex is fine. D,d,DOne,donE, etc match.