Practically, checking %ENV for certain common CGI variables will rarely steer you wrong, butt, -t is what you want. From pod (my favorite literature):

perlfunc: -t Filehandle is opened to a tty. #(STDIN by default)

perlfaq8:

How do I find out if I'm running interactively or not?
Good question. Sometimes -t STDIN and -t STDOUT can give clues, someti
+mes not.

    if (-t STDIN && -t STDOUT) {
        print "Now what? ";
    }
On POSIX systems, you can test whether your own process group matches 
+the current process group of your controlling terminal as follows:

    use POSIX qw/getpgrp tcgetpgrp/;
    open(TTY, "/dev/tty") or die $!;
    $tpgrp = tcgetpgrp(fileno(*TTY));
    $pgrp = getpgrp();
    if ($tpgrp == $pgrp) {
        print "foreground\n";
    } else {
        print "background\n";
    }
[download]

and even perlsyn:

    $on_a_tty = -t STDIN && -t STDOUT;
    sub prompt { print "yes? " if $on_a_tty }
    for ( prompt(); <STDIN>; prompt() ) {
        # do something
    }
[download]

 
___crazyinsomniac_______________________________________
Disclaimer: Don't blame. It came from inside the void
perl -e "$q=$_;map({chr unpack qq;H*;,$_}split(q;;,q*H*));print;$q/$q;"