Thanks for your very insightful answer ! PerlMonks is such a great place because people like you take the time to research into questions and give the great replies as this :-)

A few points:

My machine is Win2k on P4 1800 GHz and 256 MB of memory.
The file loading takes 22 seconds. The first search method (loop with substr takes 2 minutes), the second takes 1 second. What concerns me though, is that neither reported that the sequence "wasn't found" in a completely random binary file (the probability of 0xff x 16 to appear is 2E-128, quite unlikely).
I wonder about the speed differences... what makes my program run slower on a far stronger PC ? Bad Perl implementation (Activeperl 5.6) ?

Also, could you please elaborate on the use of the following line:

$/ = \(100*1024*1024)

I only used $/ as in "undef $/" to read a file wholly, or to set a record separator. What does your line do ?

----------- Update ------------

I think I figured out the performance problem. My PC usually runs at 190 MB memory usage, which means that the 100MB slurped threw it into the "virtual realm", which naturally downgrades performance.
Now I read as follows: 

until (eof(FH))
...
read(FH, $val, 128);
...

[download]
And then just compare to the wanted value. The whole process (which now combines reading & testing) takes 28 seconds.

By the way, I was wrong about other thing. You first method (with substr) works correctly - it doesn't find the string. The second method (with index) seems to do find a string, or at least it thinks so, which is probably wrong (look at your test output, it's obvious there too)

In reply to Re: Re: is perl the best tool for this job?(emphatically Yes!) by spurperl
in thread is perl the best tool for this job ? by spurperl

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.