Have I fundamentally misunderstood anything

Yes, the three main tricks , but they're not exactly easy to spot unless you're familiar with them :)
1) one single correct unpack call with a clever template is faster than everything else
2) slurping the entire file into one large string is faster than reading byte by byte (its how hard disks work)
3) substr-ing accross a string is faster than chopping a string (or copying a string then chopping it ...
aliasing and/or pass-by-reference is faster than copying

What you did is replace unpack with substr+unpack -- two operations with one -- this will be slower

one of the slow things about your original program is using oct/hex+unpack -- unpack can do most things by itself , see Re: ID3v2 TAG Footer Reading goes wrong (more subs),Re: hex to binary ( UInt32 / Int32 )

reducing the number of calls speeds things up

In reply to Re^3: Reading binary file performance by Anonymous Monk
in thread Reading binary file performance by oneill

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.