Problem

You want to know whether your program is being called interactively or not. For instance, a user running your program from a shell is interactive, whereas the program being called from cron is not.

Solution

Use -t to test STDIN and STDOUT:

sub I_am_interactive {
    return -t STDIN && -t STDOUT;
}

If you're on a POSIX system, test process groups:

use POSIX qw/getpgrp tcgetpgrp/;

sub I_am_interactive {
    local *TTY;  # local file handle
    open(TTY, "/dev/tty") or die "can't open /dev/tty: $!";
    my $tpgrp = tcgetpgrp(fileno(TTY));
    my $pgrp  = getpgrp();
    close TTY;
    return ($tpgrp == $pgrp);
}

Discussion

The -t operator tells whether the filehandle or file is a tty device. Such devices are signs of interactive use. This only tells you whether your program has been redirected. Running your program from the shell and redirecting STDIN and STDOUT makes the -t version of I_am_interactive return false. Called from cron, I_am_interactive also returns false.

The POSIX test tells you whether your program has exclusive control over its tty. A program whose input and output has been redirected still can control its tty if it wants to, so the POSIX version of I_am_interactive returns true. A program run from cron has no tty, so I_am_interactive returns false.

Whichever I_am_interactive you choose to use, here's how you'd call it:

while (1) {
    if (I_am_interactive()) {
        print "Prompt: ";
    }
    $line = <STDIN>;
    last unless defined $line; 
    # do something with the line
}

Or, more clearly:

sub prompt { print "Prompt: " if I_am_interactive() }
for (prompt(); $line = <STDIN>; prompt()) {
    # do something with the line
} 

See Also

The documentation for the standard POSIX module, also in Chapter 7 of Programming Perl; the -t file-test operator in Chapter 3 of Programming Perl and in perlop (1)