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
