My XML file looks something like this:
Assuming that your XML is well formed (it isn't) you can use a number of CPAN modules for the job. Most of the will involve parsing the document, finding your bit with an XPath query, and printing the document out again.
Here's an example with XML::Twig. I had to fix up the XML to get it to parse.
#!/usr/bin/perl
use strict;
use warnings;
use XML::Twig;
my $twig = XML::Twig->new(
twig_handlers => {
'conf/prop' => sub { $_->{att}{val} =~ s/a.org/b.org/; }
},
pretty_print => "indented"
);
$twig->parse(join "", );
$twig->print;
__END__