# in your parsing routine:
    my $parser = XML::LibXML->new();
    local $XML::LibXML::match_cb = \&match_uri;
    local $XML::LibXML::open_cb = \&open_uri;
    local $XML::LibXML::read_cb = \&read_uri;
    local $XML::LibXML::close_cb = \&close_uri;
    my $doc = $parser->parse_file($file);
    $doc->process_xinclude();
    my $style_doc = $parser->parse_file($stylesheet);
    my $stylesheet = XML::LibXSLT->parse_stylesheet($style_doc);
    my $results = $stylesheet->transform($doc);

 ...

# the sub definitions:
# NB: This is just one example using a string - the
# return from open_uri is an opaque type, so you can
# use a file handle if that's appropriate
sub match_uri {
    my $uri = shift;
#    warn("match: $uri\n");
    return $uri !~ /^\w+:/; # only handle URI's without a scheme
}

sub open_uri {
    my $uri = shift;
#    warn("open: $uri\n");
    my $str = some_function_to_get_the_uri($uri);
    return $str;
}

sub close_uri {
}

sub read_uri {
    return substr($_[0], 0, $_[1], "");
}