use strict;
use warnings;

use List::Util qw( reduce );

print factorial(5), "\n";

sub factorial {
  return reduce { $a * $b } 1 .. $_[0] || 1;
}
__END__
__OUTPUT__
Name "main::a" used only once: possible typo at mytest.pl line 10.
Name "main::b" used only once: possible typo at mytest.pl line 10.
120