recursive calculation of factorials

#!/bin/perl
#factorials from standard input
sub factorial{
    if(@_ != 1){
        print "Warning! More/Less than one parameter to the factorial subroutine";
    }
    if ($_[0] == 1) {
        return 1;# force a return to go back up the stack
    }
    $_[0] * factorial($_[0]-1);# no need for the return keyword, last thing done so at the top of the stack
}

while (<>) {
    chomp;
    $factorialn = &factorial($_);
    print "Factorial " . $_ . " or " . $_ . "! is equal to " . $factorialn . "\n";
}