And at least a few more ways of which I don’t doubt mr_mischief was aware. :P

~>perl -MCGI -e 'print CGI->header'
Content-Type: text/html

~>perl -MCGI -e 'print CGI->header("text/plain")'
Content-Type: text/plain

~>perl -MCGI=:standard -e 'print header("text/plain; charset=UTF-8")'
Content-Type: text/plain; charset=UTF-8
[download]

(Update: chopped p rint fixed.)