my $ref = $dbh->selectall_hashref(...
for my $row (values %$ref) {
   $row = [ @$row{qw/label subject/} ];
}
# or
# $_ = [ @$_{qw/label subject/} ] for values %$ref;
# or
# $ref = { map {; $_, [ @{$ref->{$_}}{qw/label subject/} ] } keys %$ref };