I think that your looking at this from the wrong angle.

I would say the my, our, package and sub all declare something. They declare a name.

The fact that in some cases, that name will refer to a piece of storage that may already exist our, or a new piece of storage that will be created my, or to a potentially pre-existing, namespace in which other names can be grouped package, or a name of a piece of code, that will exist within current, potentially implicitly named, namespace sub, could be considered a side-effect of the declaration to the compiler that I wish to use a particular name at the current level of scope.

This is further reenforced on those occasions when my doesn't cause a new piece of memory to be allocate. Eg.

for my $key ( keys %hash ) {...}
while( my $line = <FILE> ) {...}
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In both these cases, my isn't creating anything. It is declaring a name that is used to refer to a pre-existing piece of storage for the duration of a level of scope.

In all cases, the four keywords declare names that the programmer wishes to use for some entity. That sometimes that entity may be a pre-existing one and other cases it will be created is (almost) incidental.

The joker in the pack of course is local, which says I wish to use a pre-existing name to refer to a new piece of storage.