It would have been well to explain what you want, instead. I'm not sure what you're trying to accomplish. It looks like you want to assign a slice to a slice inside a map in void context. But, you are assigning the value of a hash of hashes to the number of elements in an array reference, instead.

Your hash slices ought to look like @foo{LIST}. Map returns the list it produces, so you may clarify things for yourself by rewriting the map in the form,     @{slice_or_whatever} = map {whatever} @results;

Your C-style for loop is confusing things, too.

for (@{$sth->fetchall_arrayref}) {
# . . .
}
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will suffice.

As always, use strict; and use warnings; will keep you out of trouble.