#!/usr/bin/perl -w
use strict;

my $log_string = "warning2 . . .";
my $logfile = "log.txt";
my @entries = ();
my $entry;

open LOG, "< $logfile" or die "cannot read $logfile";
@entries = <LOG>;
map { chomp } @entries, $log_string;
unshift @entries, $log_string;
close LOG;

open OUT, "> $logfile" or die "cannot write to $logfile";
foreach $entry ( @entries[0..9] ) {
print OUT $entry."\n";
}

close OUT;