CGI.pm does this by checking the definedness $ENV{'REQUEST_METHOD'}. You could also go further by checking -t STDIN, though if you're not running on an interactive terminal, you'll get different results. (That may be what you want.)

In reply to Re: Outputting to browser vs DOS...how can you tell? by chromatic
in thread Outputting to browser vs DOS...how can you tell? by S_Shrum

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