One alternative to an explicit loop is to use a hash slice to set multiple keys at once. If you are not using the values, only using a hash to count unique keys, you can even assign an empty list. Looks like this:

# yes, the %hash gets a @ in front to show that we are going to use mu
+ltiple keys; perl knows it's really %hash being used instead of @hash
+ because of the {}

@hash{KEYLIST} = ();

print "unique keys: ", scalar keys %hash;
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where KEYLIST can be any expression returning a list, e.g. split ...