You're doing too much work, when you can just use Memoize and take advantage of the Memoize::Storable capability.

package Audio::Scrobbler::SimilarArtists;
use strict;
use Memoize;

my %cache;

sub new {
   my ($class, %parameters) = @_;

   my $self = bless ({}, ref ($class) || $class);
   my %options = (
      min_match  => 75,
      cache_time => '1 week',
      cache_file  => '/tmp/audioscrobbler.cache',
      %parameters,
   );

   $self->{'_options'} = \%options;
   $self->{cache}      = \%cache;

   tie %{ $self->{cache} } => 
     'GDBM_File', $self->{_options}->{cache_file}, O_RDWR|O_CREAT, 066
+6;
   memoize 'lookup', SCALAR_CACHE => [HASH => \%cache];
   
   return $self;
}

sub lookup {
   # same as above, but without caching code: memoize handles that.
}
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