If you have a while loop that is spinning and you don't want to prompt the user for any key to be pressed such as with <STDIN> and you don't want to use Term:ReadKey module but to silently break out when she hits ENTER key use the following 3 line function called "keystroke()" where $j variable will change from 0 to 1 when the ENTER key is pressed:

while (true) {
    ...
    if (&keystroke == 1) {
        last;
    }
    ...
}

sub keystroke {
    $i = '';
    vec($i, fileno(STDIN), 1) = 1;
    $j = select ($i, undef, undef, 0);
}