Thanks for the pointer to

https://metacpan.org/module/SOAP::Server#SOAP::Server::Parameters

So, essentially all I have to do is add the following to the top of my module and it adds a SOAP::SOM object to the variables passed to any method called as a SOAP service.

use vars qw(@ISA);
@ISA = qw(Exporter SOAP::Server::Parameters);
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The additional variable allows access to the envelope which has a whole load of useful stuff in there including access to the request. And access to the request can be done like in the following example

sub mymethod{
    my $envelope = pop(@_);
    
    my $sample_id = $envelope->dataof("//sample_id")->value if(defined
+ $envelope->dataof("//sample_id"));
    my $match_id = $envelope->dataof("//match_id")->value if(defined $
+envelope->dataof("//match_id"));
}
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This is great as that means I can do my own verifying of the request data.

Thanks again for the pointer.