With proper formatting, your question would've been comprehensible:

Hi Monks, I am new to perl, so my question can be stupid question. I have file like this:

1   3    5
2   7    6
3   10   7
[download]

I want to subtract 2nd column of each line from previous line's 2nd column. I want to have output like this:

1   3   5
2   4   6
3   3   4
[download]

Any help will be appreciated.

To solve your problem, just store the current value at the end of every iteration, so you still have it in the next iteration:

#!/usr/bin/perl
use strict;
use warnings;

my $prev_2 = 0;

while (<DATA>) {
    my ($c1, $c2, $c3) = split;
    printf "%3d %3d %3d\n",
            $c1, $c2-$prev_2, $c3;
    $prev_2 = $c2;
}

__DATA__
1   3    5
2   7    6
3   10   7
[download]

Output:

  1   3   5
  2   4   6
  3   3   7
[download]

(BTW, your desired output suggests that you want to subtract the previous value from the current one, not the other way round).