You don't have to worry about the data staying with the identifier since that association is saved in the hash for the line.

Re-formatting the lines is just looping over @list:

my @keys = ...order you want the named keys in...

for my $h (@list) {
  print join(",", $h->{first}, $h->{date},
            map { $_." ".$h->{$_} } @keys), "\n";
}
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