The basic technique for extraction and substraction was shown by toolic. I don't know why you haven't used this nor why you seem to be asking exactly the same question in response to his reply: he's shown you how to extract the data, do the subtraction and output the result in the formats requested. Perhaps you should provide expected output to clarify your question.

Now that we can see the format of your input data, I can show you how to read pairs of lines.

#!/usr/bin/env perl

use strict;
use warnings;

{
    local $/ = "inst2\n";

    while (<DATA>) {
        /^(\w+)=\w+\s+(\w+)/;
        printf "%#x %1\$d\n" => hex($2) - hex $1;
    }
} 

__DATA__
0x405d75=inst1
0x405d7a=inst2
0x4035f7=inst1
0x4035f8=inst2
0x4035f8=inst1
0x4035fb=inst2
[download]

Output:

0x5 5
0x1 1
0x3 3
[download]