Your code contains at least one serious bug: 
my $i = index $block, "\n";
while($i > 0){

    push @lines, substr($block,0,$i);
    substr($block,0,$i+1,'');
    $i = index $block, "\n";

}

[download]
 What if your data contains an empty line? At that point, index $block, "\n" will at one time return 0. From there on, the loop's condition will always return 0, $left will grow until the end of the file, and the data will not ever be pushed onto the array @lines.

What if the file data doesn't end with "\n"? Again: the last line is disregarded. At least, push the contents of $left onto the array, if it has a length > 0.

The next code will behave better on both regards, though I haven't benchmarked it, but it could be that your suspiciously excellent results are indeed partly caused by the presence of an empty line. 

my $block;
local $_ = '';
my $i;
while(my $n = read $fh, $block, 8192 ) {

    $_ .= $block;

    while((my $i = index $_, "\n") >= 0){
        push @lines, substr($_, 0, $i);
        substr($_, 0, $i+1, '');
    }
}
push @lines, $_ if length;

[download]

In reply to Re: Why use <$fh> at all? (bug fix) by bart
in thread Why use <$fh> at all? by cluka

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.