It appears ways to do this are numerous and listed, but i believe there is one thing being overlooked in your original attempt. The code :

($user, $foo{$user}) = split /:/, $_, [2];
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The problem here is that split() returns and array onto the stack, so i don't believe that you can get $user and use $user in the same array assignment. While there is more than one way to do it, I would look at the above for working code. As for the idea of this specific "syntactic sugar", i don't think i quite works.

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may the foo be with you