My guess is that the header isn't being sent but that you have named your CGI *.xls and IE is ignoring the header and interpretting the downloaded text as a spreadsheet and silently deciding to convert it to such.

Update: Try it with a different browser. Or better, use LWP::Simple to see what is really being sent:

perl -MLWP::Simple -e "print get('http://www.alienfishexchange.com/')"
+ | more
perl -MLWP::Simple -e "print join '|',head('http://www.alienfishexchan
+ge.com/')"
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        - tye (but my friends call me "Tye")