hm, i would suggest to do something like

&capitalise( \%horoscopes );

sub capitalise {
  my $values = shift;
  foreach ( keys ( %{$values} ) ) {
  ....
...this is not tested...
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because i think you want to give your sub a reference on the hash %horoscopes. but what you actualy did is to store a reference in $reference and give it to the sub but you used the scalar reference from the main program.
you should not only read the reftut again but read perlsub too.