my $full_rs = $schema->resultset("user");
my @ids = $full_rs->get_column("id")->all;

# -or-

my $simple_rs = $full_rs->search({},
                                 { columns => [qw/ id /] });

print $_->id, $/ for $simple_rs->all;

# -or, no objects-

# use parent "DBIx::Class::ResultSet::HashRef"; <- in resultset class.
my $ids = map { $_->{id} }
      $schema->resultset("user")
             ->search({},{columns => ["id"])
             ->hashref_rs
             ->all;