Every program running on Unix can check if its input/output is connected to a tty or not. In Perl, this is done via

if(-t STDOUT) {
    print "tty\n";
} else {
    print "no tty\n";
}
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If you run this program from the command line, it'll print tty. If you run it via a remote shell on another host, you'll see no tty.

I suspect your program is checking this and prints an error if it finds out about it.