This mission requires basic knowledge of linux shell commands and operators. The command "ls" lists the contents of a directory, while the semicolon (";") character ends one command without requiring a newline / RET.
You need to end the "cal" command and run ls. The script being run is similar in effect to
echo exec("cal ... ".$_POST['cal']);
With a normal input (a 4 digit number) the code is effectively
echo exec("cal ... 1999");
We want to end the "cal" command and start the "ls" command, so we use a semicolon, making the input "1999; ls" and the code
echo exec("cal ... 1999; ls");
This should output the normal calendar information, and at the bottom there should be something like
. .. level7.php cal.pl index.php k1kh31b1n55h.php perl5.8.9.core
Go to /basic/7/TheObscureFileYouJustFound.php . There's your password!