It means that the output is not well formed. This can either be: i) you made a mistake in the response header, which needs to start with Content-type: text/html and two line breaks (but proper usage of any CGI module will take care of that); ii) something is erroneously printed into your response before this header (a debug print for example).

In reply to Re^2: Debugging CGI/PERL by rvosa
in thread Debugging CGI/PERL by Trihedralguy

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