• the attr key in the constructor, is waiting for a HASHREF not a ARRAYREF.
• the constructor lacks some keys.

my $link = WWW::Mechanize::Link->new(
    {   
        url  => 'http://www.website.com/report/runreport/30',
        text => 'report',
        name => '', 
        tag  => 'a',
        base => 'http://www.website.com',
        attr => {
            'fields[xxx.name]'    => 'a name',
            'fields[xxx.version]' => '5.34'
        },
    }   
);

__END__
attr => $attr_href
[download]

hth,
PooLpi