I did something like this:

our $stdin;
our $origMode;
our $isWindows = 0;
eval q{
use Win32::Console
$stdin = new Win32::Console STD_INPUT_HANDLE;
$origMode = $stdin->Mode();
$isWindows = 1;
};

sub getPassword
{
    my $pr;
    my $required;

    if (@_)
    {
        $pr = shift || "Password";
        $required = shift || undef;
    }
    else
    {
        $pr = "Password";
        $required = 1;
    }

    my $password;

    until ($password)
    {
        print $pr, ": ";

        # if we don't have Term::ReadKey, skip it.
        my $isReadmode = 0;
        eval q{
            use Term::ReadKey;
            ReadMode(2);
            $isReadmode = 1;
        };
        # if that didn't work, try Windows.
        eval q{
            $stdin->Mode(ENABLE_LINE_INPUT | ENABLE_PROCESSED_INPUT | 
+ENABLE_MOUSE_INPUT);
        } if $@ and $isWindows;

        chomp($password = <STDIN>);

        if ($isReadmode)
        {
            eval {
                ReadMode(0);
                print "\n"; # only needed if readmode was successful.
            };
        }
        elsif ($isWindows)
        {
            $stdin->Mode($origMode);
        }

        last unless $required; # if not required, just skip checks.
        unless (length ($password) > 5)
        {
            $password = undef;
            print "Too short. ";
            next;
        }
    }

    $password;
}
[download]

Not quite foolproof, but it was sufficient for me. As you can see, I got this working (good enough) on both unix and windows - but never tried Mac, mainframes, or PDAs. YMMV.