use Data::Dumper;

## thanks Ovid :)
my @data = (
    {season => 1, ep => 1, title => 'Hellmouth' },
    {season => 1, ep => 2, title => 'Harvest'   },
    {season => 1, ep => 3, title => 'Witch'     },
    {season => 1, ep => 4, title => 'Teacher'   },
    {season => 1, ep => 5, title => 'First Date'},
    {season => 2, ep => 13, title => 'Bad'      },
    {season => 2, ep => 14, title => 'Assembly' },
    {season => 2, ep => 15, title => 'School'   }
);


my $appearances = [];
for my $ref (@data) {
  my $s = \$appearances->[ $ref->{season} - 1 ];
  $$s->{season} = $ref->{season};
  push @{ $$s->{ eps } },
       { title => $ref->{title} };
}

print Dumper $appearances;
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