You're presumably calling it on the wrong object type.  What does your code/xml look like?

Here's a simple example using getFirstChild that works:

#!/usr/bin/perl
use strict;
use warnings;
use XML::LibXML;

my $parser = XML::LibXML->new();
my $doc = $parser->parse_fh(\*DATA);

for my $elem ( $doc->getElementsByTagName('foo') ) {
    my $val = $elem->getFirstChild()->nodeValue();

    print "$val\n";   # Foo1 Foo2
}  

__DATA__
<?xml version = "1.0" encoding="UTF-8" ?>
<doc>
  <foo>Foo1<bar>Bar</bar></foo>
  <foo>Foo2</foo>
</doc>
[download]