G'day sumathigokul,

Your regex baffles me! You're trying to match (and capture) one or more digits immediately preceded by and followed with a colon — clearly not what you want. I'd recommend spending some time with the Perl Regular Expressions Tutorial.

You have no code to output the wanted lines — which is why that part isn't working. Take a look at open; note how both of the initial examples (for reading and writing) use the 3-argument form and lexical filehandles; start doing this yourself.

This code has the logic you need — you'll need to add the I/O:

#!/usr/bin/env perl

use strict;
use warnings;

my $re = qr{ \A \s+ ( \d+ ) }x;
my $high = 0;
my @lines;

while (<DATA>) {
    /$re/ or next;
    my $num = $1;
    next if $num < $high;

    if ($num > $high) {
        $high = $num;
        @lines = ();
    }

    push @lines, $_;
}

print "Highest number: $high\n";
print @lines;


__DATA__
High fanout nets in the post compile netlist:
    Fanout  Type          Name
    --------------------------
    2       INT_NET       Net   : c_c
                          Driver: c_pad
    2       INT_NET       Net   : b_c
                          Driver: b_pad
    2       INT_NET       Net   : a_c
                          Driver: a_pad
    1       INT_NET       Net   : sum_c
                          Driver: sum_1_SUM0_0
    1       INT_NET       Net   : N_5
                          Driver: sum_1_CO0_i
[download]

Output:

Highest number: 2
    2       INT_NET       Net   : c_c
    2       INT_NET       Net   : b_c
    2       INT_NET       Net   : a_c
[download]