The documentation of the post method of LWP::UserAgent shows

my $res = $ua->post( $url, $field_name => $value, Content => $content 
+);
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Your code uses

$ua->post(
    $self->{'auth'}->api_url_base . 'shares',
    Content => encode_json($json),
    $header,
    );
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So, dereference the header (I also tried to moving it before Content, but it doesn't seem to matter). This should remove the hash reference from the request.

$ua->post(
    $self->{'auth'}->api_url_base . 'shares',
    %$header,
    Content => encode_json($json)
);
[download]

The ::std_case field just handles standard lowercasing of header keys, nothing to worry about.