With about 10K keys, this might not be a killer issue, but you might notice a difference:

 foreach my $key (keys %hash_db) # creates a list in memory 
                                 # containing all key strings

# vs.

 while ( my ($key, $val) = each %hash_db )
                # iterates over the DB entries one at a time
                # and doesn't hold all keys in memory at once
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