Update: So I'd say that the unkosher '(' comes from the opening paren in 'HTTP::Request=HASH(0x1c23960)'

You were spot on with the stringification causing the problem. I was trying to put some whitespace between the various Dumper outputs so wrote a rather stupid line of code:

print Dumper %$request . "\n";
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Culprit found!!! Sorry for the senior moment!

I think you are right about the source of the opening parenthesis. But I am using an existing module, LWP::Authen::OAuth2 that works. I am only adding a Dumper statement to it to try and understand what it needs that I am not giving it. It should take a hashref. This is the code I am using to call it:

open my $fh, '<', "linkedin.json";
my $json;
{
    local $/;
    $json = <$fh>;
}
close $fh;

$params = decode_json($json);

my $header = {
    'X-Restli-Protocol-Version:' => '2.0.0',
};

print Dumper $params;
print "\n\n============\n\n";
my $res = $linkedin->post('https://api.linkedin.com/v2/ugcPosts', $par
+ams, $header);
if ($res) {
    print Dumper $res;
    exit 0;
}
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There is an external JSON file so that I can use exactly the example given in the LinkedIn API documentation without introducing data structure errors. The only thing that's changed from that example is the person:id value.

I'm beginning to think that the LinkedIn API is sufficiently recalcitrant to warrant writing a module from scratch for posting to it. Just use the LWP::Authen::OAuth2::ServiceProvider subclass for authentication and not for posting.