Here's a more efficient idiom for grabbing a single key from a hash reference:
my $key = each %{$hashref};
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.
Dan Kubb, Perl Programmer
Updated: Made minor corrections noted by Limbic-Region