Q:

How can I extract child values from XML with Perl's XML::Twig? 

I am parsing the XML file and trying to access the values in XML file. 
#! /usr/bin/perl -w

use strict;
use XML::Twig;

my $file = 'files/camelids.xml';
print "File :: $file\n";
my $twig = XML::Twig->new();

$twig->parsefile($file);
# print "twig :: $twig\n";

my $root = $twig->root;
# print "root :: $root\n";

my $num = $root->children('species');
print "num :: $num\n\n\n";

print $root->children('species')->first_child_text('common-name');

sample XML file is:
<? xml version="1.0"? >
<camelids>
  <species name="Camelus bactrianus">
    <common-name>Bactrian Camel</common-name>
    <physical-characteristics>
      <mass>450 to 500 kg.</mass>
      <appearance>
          <in-appearance>
              <inside-appearance>This is in inside appearance</inside-appearance>
          </in-appearance>  
      </appearance>
    </physical-characteristics>
  </species>
</camelids>

Output is:
File :: files/camelids.xml
num :: 1

Can't call method "first_child_text" without a package or object reference at xml-twig_read.pl line 19. 

How to fix this issue?  
Is there anything wrong in this line of code and any modification needed (here I am trying to get the common-name as Bactrian Camel)
print $root->children('species')->first_child_text('common-name');

A:

Change the last lines to
my @nums = $root->children('species');
print "num :: @nums\n\n\n";

foreach my $num (@nums) {
print $num->first_child_text('common-name');
}

children returns an array, so you need to run over it. 
To help debugging, try this:
my @nums = $root->children('species');
use Data::Dumper; #More debug information like this than a normal print
print Dumper @nums;

foreach my $num (@nums) {
print $num->first_child_text('common-name');
}

