It is also a good practice to check for the existence of hash keys, particularly when prompting for input via the keyboard:

if (exists $locs{$loc}) {
    # do something...
}
else {
   die "key $loc does not exist";
}
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Update: I should have read mscharrer's node more closely, as this was already mentioned. But, I provided the link to the docs, so I guess this is not completely redundant.