Here's a snippet that I think does what you mean?

# assuming that...
my $string = "Perl Monks";
my $n = length($string);
my $i = 1;
my $result = 0;

# here it is.

$result += ord($_) * 31^($n-$i++) for split '', $string;
        ^   ^   ^         ^  ^ ^             ^    ^
        |   |   |         |  | |             |    +-- String to split
        |   |   |         |  | |             +-- split all the charact
+ers
        |   |   |         |  | +-- post increment $i
        |   |   |         |  +-- $i is the index
        |   |   |         +--- $n is the length of $string
        |   |   +--- this is the character returned from split   
        |   +--- ord() returns the ascii code of the character
        +--- add what we have to $result
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