This loops through the lines, getting the Fanout value from the lines with INT_NET, and if it's equal to or greater than the maximum Fanout value so far, it saves that line along with its Fanout value as a 2-element array reference in the array @lines.

Once it's done, it has the largest Fanout value in $max and all the lines with that value (along with some other lines, possibly) as the first elements in the array @lines. The map/grep statement goes through those, grepping for the ones with the $max Fanout value and then using map to pull out the line itself and pass that to print.

#!/usr/bin/env perl
use 5.010; use warnings; use strict;

my $max = 0;
my @lines;  # 2D array, each element holding a line and its Fanout val
+ue

while (<DATA>){
    if (/(\d+)\s+INT_NET/ and $1 >= $max ){
        push @lines, [$_,$1];
        $max = $1;
    }
}
print for map { $_->[0] } grep { $_->[1] == $max } @lines;

__DATA__
    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
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