Here's an example w/XML::Rules. Note that getting at the underlying expat parser is undocumented, but I'm sure Jenda would be willing to add something as part of the official API :-)

use strict;
use warnings;

use XML::Rules;

my @rules = (
  gibsonca => undef,
  _default => sub {
    no warnings 'uninitialized';
    return if $_[1]->{_content} =~ /\S/;
    my $p = $_[4]{parser};
    print $p->current_line(),"\n";
    return;
  },
);

my $xr = XML::Rules->new(rules => \@rules);

$xr->parse(<<XML);
<gibsonca>
<abc>fds </abc> <!-- ok -->
<ddd></ddd> <!-- not ok -->
<eee> </eee> <!-- not ok -->
</gibsonca>
XML
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