(Alan Flavell <flavell+www@a5.ph.gla.ac.uk> answers...)
The Common Gateway Interface (CGI) specifies a software interface between a program ("CGI script") and a web server (HTTPD). It is not specific to Perl, and has its own FAQs and tutorials, and usenet group, comp.infosystems.www.authoring.cgi
The original CGI specification is at: http://hoohoo.ncsa.uiuc.edu/cgi/
Current best-practice RFC draft at: http://CGI-Spec.Golux.Com/
Other relevant documentation listed in: http://www.perl.org/CGI_MetaFAQ.html
These Perl FAQs very selectively cover some CGI issues. However, Perl programmers are strongly advised to use the CGI.pm module, to take care of the details for them.
The similarity between CGI response headers (defined in the CGI specification) and HTTP response headers (defined in the HTTP specification, RFC2616) is intentional, but can sometimes be confusing.
The CGI specification defines two kinds of script: the "Parsed Header" script, and the "Non Parsed Header" (NPH) script. Check your server documentation to see what it supports. "Parsed Header" scripts are simpler in various respects. The CGI specification allows any of the usual newline representations in the CGI response (it's the server's job to create an accurate HTTP response based on it). So "\n" written in text mode is technically correct, and recommended. NPH scripts are more tricky: they must put out a complete and accurate set of HTTP transaction response headers; the HTTP specification calls for records to be terminated with carriage-return and line-feed, i.e ASCII \015\012 written in binary mode.
Using CGI.pm gives excellent platform independence, including EBCDIC systems. CGI.pm selects an appropriate newline representation ($CGI::CRLF) and sets binmode as appropriate.