Here's a more efficient idiom for grabbing a single key from a hash reference:

  my $key = each %{$hashref};
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The approach you're using with my($key) = keys %{$hashref}; essentially generates a list of the keys in memory, assigns the first element in the list to $key, and then throws rest of the list out. If the size of the hash is large this could be wasteful, but even with small hashes there is a noticeable difference between the two idioms when benchmarking.

Updated: Made minor corrections noted by Limbic-Region