First off, the XML::Checker version 0.13 documentation is wrong. Its example implies an array ref should be used with 'Handlers', but it needs a hash.

Second, XML::Checker does not find your DTD palatable. It gets upset by the ! following the c in <!ELEMENT a (b,c!)>.

With those two issues cleaned up:

use strict;
use warnings;

use XML::Checker::Parser;

my %expat_options =
  ( KeepCDATA => 1, Handlers => {Unparsed => \&my_Unparsed_handler} );
my $parser = new XML::Checker::Parser(%expat_options);

open OUT, '>', 'fail.xml';
print OUT <<OUT;
<!DOCTYPE a [
    <!ELEMENT a (b,c)>
    <!ELEMENT b (#PCDATA)>
    <!ELEMENT c (#PCDATA)>
]>
<a>
"something should not be here"
<b></b>
<c></c>
</a>
OUT
close OUT;

eval {
    local $XML::Checker::FAIL = \&my_fail;
    $parser->parsefile("fail.xml");
};

if ($@) {
    print $@;
} else {
    print "xml has been validated";
}

sub my_fail {
    my $code = shift;
    die XML::Checker::error_string( $code, @_ ) if $code < 200;
    XML::Checker::print_error( $code, @_ );
}
[download]

Prints:

XML::Checker ERROR-149: Element should only contain sub elements, foun
+d text ["something should not be here"]
    Context: line 7, column 0, byte 104
[download]