Well, as a first start, I'd try something like this: 
#!/usr/bin/perl
my $FILE = "the_log_file";
my $MAXSIZE = 2**20; # 1 meg
exit 0 unless -s $FILE > $MAXSIZE;
@ARGV = $FILE; # set diamond to file
undef $/; # slurp mode
$^I = ""; # enable in-place editing
while (<>) {
    substr($_, 0, $MAXSIZE - length) = ""; # save last portion
    s/.*\n//; # toss one line so it comes out at a line boundary
    print;
}

[download]

-- Randal L. Schwartz, Perl hacker

In reply to •Re: Limiting log file size by merlyn
in thread Limiting log file size by kardster

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.