The thing is that there are (at least sometimes) several <asset>s in each <account> so the $account->{asset} is an array ref. The problem with XML::Simple is that it's not always an arrayref.

You may use this instead:

use strict;
use XML::Rules;

sub do_something_with {
    print join(', ', @_), "\n";
}

my $parser = XML::Rules->new(
    rules => {
        _default => 'content',
        asset => 'as array no content',
        account => sub {
            my ($tag, $attr) = @_;
            foreach my $asset (@{$attr->{asset}}) {
                do_something_with($attr->{accountId}, $attr->{accountN
+ame}, $asset->{assetId}, $asset->{assetName});
            }            
        },
    }
);

$parser->parse(\*DATA);

__DATA__
<data>
<account>
    <accountId>001</accountId>
    <accountName>Account 001</accountName>
    <asset>    
        <assetId>abcd</assetId>
        <assetName>Asset abcd</assetName>
    </asset>
 ...
[download]

Or if you'd prefer a data structure containing all the data:

use strict;
use XML::Rules;

my $parser = XML::Rules->new(
    rules => {
        _default => 'content',
        'asset,account' => 'as array no content',
        'data' => 'pass no content'
    }
);

use Data::Dumper;
print Dumper($parser->parse(\*DATA));
[download]