The if construct can be soaked up in a trinary operator, too. Here's another way to organise things:

{
    my $rev_types = {
        both    => [qw/rev theater_avg num_theaters/],
        total   => [qw/rev             num_theaters/],
        loc_avg => [qw/    theater_avg num_theaters/],
        all     => [qw/rev theater_avg num_theaters day_multiple/],
    };

    sub _rev_types {
        my $key = @_ ? shift : 'both';
        exists $rev_type->{$key}?
          @{$rev_types->{$key}} :
          @{$rev_types->{'both'}};
    }
}

sub _select_fields {
    my $self = shift;
    return _rev_types( $self->request->param('rev_type'));
}
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That way the selection logic and accessor are all bundled up in their own scope where nobody can harm them.