The solution without any modules:

use strict;
use warnings;
use 5.010;

my $f = 1;
my $i = 1;
my $n = 4;
$f *= ++$i while $i < $n;
say $f;

The solution when using the reduce function of the standard List::Util module:

use List::Util qw(reduce);
my $fact = reduce { $a * $b } 1 .. $n;
say $fact;

reduce will take the first two values of the list on the right hand side, assign them to $a and $b respectively, execute the block.

Then it will take the result, assign it to $a and take the next element from the list, assign it to $b and execute the block. This step will be repeated till the end of the list.