To talk to iTunes from curl, spoof the user agent to pretend to be iTunes and set your store front to one of the legal values. Here, I set the store by passing it as a header field using curl's -H switch.
curl -s -A "iTunes/9.0.2 (Macintosh; U; PPC Mac OS X 10.2" -H "X-Apple-Store-Front: 143441-1,5" 'http://ax.itunes.apple.com/WebObjects/MZStore.woa/wa/viewTopLegacy?id=25204&popId=30&genreId=36' > itunes_app_Topcharts_paidapps