Now I think that this setup is really annoying, easpecially as I would like rewrite it so that every value is counted to represent how often a location has been added. Is there a way to add onto the old hash instead of completely rewriting it?

I'm not really sure if I understand your question, but you probably either want

%locations=(%locations, map {$_ => 1} @locations);
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or

$locations{$_}++ for @locations;
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