While Digest::MD5 is great, I've used the unpack checksum plenty of times when I've had to test checksums that were already written in data (e.g. lots of data collection hardware use 16-bit checksums in their output). First, I think the 32-bit line should be:
$checksum = unpack("%32C*", $foo) % 65535;

The '%' is just telling unpack to use the special checksum function, I believe the 'C' means to break the input into unsigned chars (it's not for checksum) and the '*' means repeat the 'C' for the rest of the input. You'll want to look at pack for better info on the template syntax the two functions share. According to the manpage, %N works for N-bits, so I have no reason to doubt that your second snippet will do 64-bit checksums, but I can't verify that.

--
I'd like to be able to assign to an luser

In reply to Re: On generating checksums and using unpack by Albannach
in thread On generating checksums and using unpack by jeffa

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.