#!/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;
}