Clarke and Stewart

After reading Josh Marshall's Talking Points Memo and his criticism of Bill Frist for calling Richard Clarke a liar, I got to thinking - what if Clarke did lie in his 2002 testimony to Congress while under oath? If his accounts to the 9/11 Commission, his 2002 testimony to Congress and what he details in his new book all differ significantly - then isn't it reasonable to infer that in at least one of the accounts Clarke isn't being truthful?

If this is the case - shouldn't the Feds go after Clarke with the same vigor in which they went after Martha Stewart? If Clarke did perjure himself - aren't his lies more harmful than Stewart's (who faces up to 20 years in jail)?