my $array_position;
my $file = "/home/user/file.txt";

open FILE, $file;

my @file = (<FILE>);

close FILE;
foreach ( @file )
{
    if ($_ =~ /line 3/)
    {
        $array_position=$i+1;
        print "Position in array = ". $array_position. "\n";
    }
    $i++;
}

for ($a=$array_position-1; $a<$array_position+3; $a++)
{
    print "Postion ".$a." in the array = " .$file[$a];
}

@array = @file;
splice @array, 0, 'something';
print @array;

##</code><code>##

line 1
line 2
fsefse line 3
line 4 ,...
line 5
line 6>