OK, it is a potential pitfall, but why would you expect a single value (or list of size 1) to get stretched automatically as big as the size of the list you're using in the hash slice? Better to do something like:

@netmask{@mymachines} =  "255.255.0.0" x scalar @mymachines;
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Which makes your intent as obvious as it could be.

Update Fastolfe reminds me that the above code won't do what I want it to. (it would generate a scalar that consists of the size of @mymachines iterations of "255.255.0.0" -- list context confuses me yet again! =) To get a list as long as @mymachines, you need to do

@netmask{@mymachines} = ("255.255.0.0") x scalar @mymachines;
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Philosophy can be made out of anything. Or less -- Jerry A. Fodor