sub substr {
        local($what,$where,$howmuch) = @_;
        unpack("x$where a$howmuch", $what);
    }

and then there's

    sub ordinal { unpack("c",$_[0]); } # same as ord()

In addition, you may prefix a field with a %<number> to indicate that you want a <number>-bit checksum of the items instead of the items themselves. Default is a 16-bit checksum. For example, the following computes the same number as the System V sum program:

    while (<>) {
        $checksum += unpack("%16C*", $_);
    }
    $checksum %= 65536;

The following efficiently counts the number of set bits in a bit vector:

    $setbits = unpack("%32b*", $selectmask);