It's not the construct of assigning an assignment to a variable that I am questioning here, I too use this all the time.

The source of my confusion were the hundreds of comment lines between the left and right-hand sides. IMHO, this is a way of making the code more unreadable - yes comments can do that! In your example (which, to be fair has only a single comment line, and is less likely to be confusing), I think it would be clearer to write:

$this->{key} = 
   my $val = blarg();  # this is an important note about the previous 
+assignment
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