why Perl complaints my script

Hi all,

I write a small script for permutation. When I use
version 1) Perl always complaint it although I get the
result. If I use version 2) it never says a word about
it. Any comments?

Thank you in advance,

Li

####version 1)###

use strict;
use warnings;

print permutation(5,2); # 5 choose 2

exit;

#subroutine permutation

sub permutation{

my ($n, $k)=@_;
my $result=1;

for ($k; $k>=1;$k--) {$result*=$n--;}#line 15
return $result;
}

#####screen output####
Useless use of private variable in void context at
fac1.pl line 15.
20


####version 2)###

use strict;
use warnings;

print permutation(5,2); # 5 choose 2

exit;

#######subroutine permutation##########

sub permutation{

my $n=shift;
my $result=1;

for (my $k=shift; $k>=1;$k--) {$result*=$n--;}
return $result;
}

###screen output####

20

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