#!/Perl/bin/perl -w

open(INFILE, "< file.log")
     	or die "Cant open file : $!";

open(OUT, "> results.txt")
     	or die "Cant open new file : $!";

use strict;
my $pattern = "Error while detecting new item";
my @history = ("\n") x 3;
while (<INFILE>) {
print OUT @history if /$pattern/;
    push @history, $_;
    shift @history;
}

print $.;
close INFILE;
close OUT;
exit;

##</code><code>##

results.txt
matched line 1
matched line 2
matched line 3
results of first match
matched line 1
matched line 2
matched line 3
results of second match
and so on...