use strict;

if ( -t STDIN ) {
    print "This is what you're supposed to do ...\n";
    exit;
}

# or 

use POSIX qw(isatty);
if ( isatty(\*STDIN) ) {
    print "This is what you're supposed to do ...\n";
    exit;
}

while (<STDIN>) {
    print STDOUT "Add something to STDOUT".$_;
}