You just need to maintain state. I use HTML::Parser directly. There are two flags. One flip flops as we enter and leave a text12 chunk. The other counts text12 chunks. You could just as easily trigger on the <span id='fulldescription'> tag.

use HTML::Parser ();
my $CHUNK = 2;  # get Nth instance of text12
$p = HTML::Parser->new( api_version => 3,
                         start_h => [ \&start, "self, tagname, attr" ]
+,
                         end_h   => [ \&end,   "self, tagname" ],
                         text_h  => [ \&text,  "self, dtext" ]
                       );
$p->parse_file(*DATA);

sub start{ 
    my ( $self, $tagname, $attr ) = @_;
    if ( $tagname eq 'div' and $attr->{class} eq 'text12' ) {
        $self->{text12}++;
        $self->{text12_item}++;
    }
}
sub end{ 
    my ( $self, $tagname ) = @_;
    $self->{text12} = 0 if $tagname eq 'div';
}
sub text{ 
    my ( $self, $text ) = @_;
    print $text if $self->{text12} and $self->{text12_item} == $CHUNK;
}
__DATA__
<div class="text12">
Chunk 1
</div>

<div class="text12">
Chunk 2
</div>

<div class="text12">
Chunk 3
</div>
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