use strict;
use warnings;
use XML::Simple;

my $xml_raw = <<XML_RAW;

<survey>
<animals srcurl="blah.whatever.blah" method="ftp">
    <fish name="barramundi" freshwater="yes" saltwater="yes">
      <river>Todd</river>
      <river>Katherine</river>
    </fish>    
    <fish name="carp" freshwater="yes" saltwater="no">
      <river>Tilbuster Ponds</river>
      <river>Maribyrnong</river>
      <river>Patterson</river>
      <river>Paterson</river>
      <river>Glenelg</river>
      <river>Murray</river>
      <river>Bunyip</river>
      <river>Campaspe</river>
    </fish>
    <fish name="yellowfin" freshwater="yes" saltwater="no">
      <river>Eucumbene</river>
      <river>Mulla Mulla Creek</river>
      <river>Burrungubugge</river>
      <river>Goobarragandra</river>
      <river>Bombala</river>
      <river>Murray</river>
      <river>Emu Swamp Creek</river>
    </fish>
</animals>
</survey>

XML_RAW

my $xml_hash_ref = XMLin($xml_raw, KeepRoot=>1);
my %xml_hash = %{$xml_hash_ref};
my ($tl_hk, $tl_hv) = each %xml_hash;
my $last_key = '';
my @key_stash = ();
my $ref_type = '';
my $fish_species = '';
my $fish_survey_dump ="";

# Just to show you how XML::Simple has structured the XML into a hash
#use Data::Dumper;
#print Dumper(\%xml_hash);

traverse_hash($xml_hash{$tl_hk}, $tl_hk);

# Print out the fish survey information that we wanted.
# I concatenated it into a scalar just for quick display purposes
print "\n\n$fish_survey_dump\n";

sub traverse_hash {
    my ($hash_val, $last_key) = @_;
    push(@key_stash, "$last_key ->");
    for my $key (keys %{$hash_val}) {
        $ref_type = ref($hash_val->{$key}) || "VALUE";
        print "$ref_type:    @key_stash $key -> ", $hash_val->{$key},"\n";
        if($ref_type eq 'HASH') {
            if($key=~/barramundi|carp|yellowfin/) {
                $fish_species = $key;
                concat("\n\n[ Survey information for: $fish_species ]:\n\n");
                concat("Saltwater:" . $hash_val->{$fish_species}{'saltwater'} . "\n");
                concat("Freshwater:" . $hash_val->{$fish_species}{'freshwater'} . "\n");
                concat("Rivers covered in survey:\n\n");
                for my $river (@{$hash_val->{$fish_species}->{'river'}}) {
                    concat("$river\n");
                }
            }
            $last_key = $key;
            # Loop through any sub hash's by calling traverse_hash() agian.
            traverse_hash($hash_val->{$key}, $last_key);
            pop(@key_stash);
        }elsif($ref_type eq 'ARRAY') {
            # Array reference
            traverse_array($key, @{$hash_val->{$key}});
        }else{
            # Hash value;
            # ...
        }
    }
}

sub traverse_array {
    my ($key, @array) = @_;
    for my $array_val (@array) { 
        print "ARRAY-VAL:    @key_stash $key -> ", $array_val,"\n";
        if(ref($array_val) eq 'HASH') {
            traverse_hash($array_val, undef);
        }
    }
}

sub concat {
    my $string = $_[0];
    $fish_survey_dump .= $string;
}