# assuming the file is already opened and has a <IN> filehandle

my @temp;

# and assuming that you won't match before the first 5 lines..

while (my $line = <IN>) {
	push @temp, $line;
	shift @temp if (scalar @temp > 5);
	if ($line =~ /foo/) { 
		# we've got a match.
		print $temp[0];
	}
}