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    my $ua = WWW::Mechanize->new();
    my $r = HTTP::Request->new(
...
        '... that file body ...'
    );
    my $res = $ua->request( $r,  );

    my $ua = WWW::Mechanize->new();
    $ua->credentials('apikey','that_api_key');
...
        $file_content
    );
    my $res = $ua->request( $r );