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unpack TEMPLATE,EXPR

Unpack() does the reverse of pack(): it takes a string representing a structure and expands it out into a list value, returning the array value. (In scalar context, it returns merely the first value produced.) The TEMPLATE has the same format as in the pack() function. Here's a subroutine that does substring:

sub substr {
    my($what,$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);