It is syntactically valid, it just won't do what he wants (no chance in hell).

The expression will evaluate to $xml->{{}}, and that key (the stringified version of that anonymous hashref) is not defined, so a warning will be thrown.
something like that ;)(actually $xml->{gene}{{}} , but it's not really important)

If you didn't know what the comma operator would do in this expression, read "List" is a Four-Letter Word, as well as perldoc perlop

update:

use Data::Dumper;
local $\="\n";

$a = {
    gene => { id => 3, label => 'generrrous' }
};

print $a->{ gene, id}=1;
print $a->{ gene => id };
print Dumper $a;

__END__
1
1
$VAR1 = {
          'gene' => {
                      'label' => 'generrrous',
                      'id' => 3
                    },
          'gene‡˜id' => 1
        };
[download]

MJD says you can't just make shit up and expect the computer to know what you mean, retardo! ** The Third rule of perl club is a statement of fact: pod is sexy.

If I want to access something nested within the gene tags. For example, <gene id = "1"><gene_seq id = "1"></gene_seq></gene>. How do I do that?

    print $xml->{gene_seq}{$id}{'startpos'}, "\n";
[download]

Does not do it.
Read more... (1196 Bytes)

#!/usr/bin/perl -w

use strict;
use warnings;
use XML::Simple;
use Data::Dumper;

my $data = do {local $/;<DATA>};
my $xml  = new XML::Simple(keeproot => 2);# this is what you want
$xml  = XMLin($data);

print Dumper($xml), "\n\n";

for my $id (keys %{ $xml->{gene} }) {
  # print $xml->{gene}{$id}{'label'}, "\n";
   # print $xml->{gene_seq}{$id}{'label'}, "\n";
    print $xml->{gene}{$id}{gene_seq}{'startpos'}, "\n";
#print "$xml->{'gene'}{'id'}, $xml->{'gene'}{'label'}\n";
}

__DATA__
<?xml version="1.0" ?>
<many_genes>
 <gene id = "1" label = "gene_of_interest">
  <gene_seq id = "1" startpos = "5999"/>
 </gene>
 <gene id = "2" label = "Another_gene_of_interest">
  <gene_seq id = "2" startpos = "96819"/>
 </gene>
</many_genes>
[download]

yields

$VAR1 = {
          'gene' => {
                      '1' => {
                               'gene_seq' => {
                                               'startpos' => '5999',
                                               'id' => '1'
                                             },
                               'label' => 'gene_of_interest'
                             },
                      '2' => {
                               'gene_seq' => {
                                               'startpos' => '96819',
                                               'id' => '2'
                                             },
                               'label' => 'Another_gene_of_interest'
                             }
                    }
        };


5999
96819

It's like a tree ;)

# you start with
$xml->{gene}->{1}->{gene_seq}->{startpos};
# and you make it variable
$xml->{gene}->{$id}->{gene_seq}->{startpos};
# and its same as
$xml->{gene}{$id}{gene_seq}{startpos};
$$xml{gene}{$id}{gene_seq}{startpos};
$$xml{gene}->{$id}{gene_seq}->{startpos};

MJD says you can't just make shit up and expect the computer to know what you mean, retardo! ** The Third rule of perl club is a statement of fact: pod is sexy.