I don't know how to tell what href is relative to, but even just to get the value of href is a bit gimpy for "processing instructions", which is what <? ... ?> are.

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

my $parser = XML::LibXML->new;
my $doc = $parser->parse_string(<<'EOX');
<?xml version="1.0" encoding="UTF-8"?>
<?xml-stylesheet type="text/xsl" href ='abc"efg'?>
<_/>
EOX

foreach my $node ($doc->findnodes('//processing-instruction()')) {
    my $name = $node->nodeName;

    if ($name eq 'xml-stylesheet') {
        # getData is a string like q{type="text/xsl" href="/test.xsl"}
        # which is what makes it annoying
        my $attr_str = $node->getData;
        # manually parse the string like href='abc"efg';
        # there might be a better way of doing this
        $attr_str =~ m{href\s*=\s*(['"])([^\1]+)\1};

        my $href = defined $2 ? $2 : '';
        print "$name href: >>>$href<<<\n";
    }
}
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