use strict;
use warnings;
use Data::Dumper;
$Data::Dumper::Indent = 1;

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;
my $curr_season = 0;
foreach my $data (@data) {
    my $record = { title => $data->{title} };
    if ($data->{season} != $curr_season) {
        push @appearances => {
            season => $data->{season},
            eps    => [ $record ]
        };
        $curr_season = $data->{season};
    }
    else {
        push @{$appearances[-1]{eps}} => $record;
    }
}

print Dumper \@appearances;