kcott's version works as you expect. Your modified version (writing lines in a loop to another file) does work here, though output is not what you want.
Run first as perl program.pl -BUG_MODE=0 (kcott's version); then as perl program.pl -BUG_MODE=1 (your version) ...
#!/usr/local/bin/perl -s
use warnings; use strict;
our $BUG_MODE;
printf "** Running in bug mode: %s **\n\n" , map uc( $_ ) , $BUG_MODE
+? 'yes' : 'no';
my $re = qr{ \A \s+ ( \d+ ) }x;
my $high = 0;
my @lines;
my $filtered;
my $in_fh = \*DATA or die qq[Cannot open DATA: $!];
open (my $out_fh, '>' , \$filtered ) or die qq[Cannot open \$filtered
+to write: $!];
while (<$in_fh>) {
/$re/ or next;
my $num = $1;
next if $num < $high;
if ( $num > $high ) {
$high = $num;
@lines = ();
}
push @lines , $_;
$BUG_MODE
and print $out_fh @lines;
}
close ($in_fh);
print "Highest number: $high\n";
$BUG_MODE
or print $out_fh @lines;
close ($out_fh);
printf "Filtered output ...\n%s\n" , $filtered ;
__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
# Output.
perl x.pl -BUG_MODE=0 ; perl x.pl -BUG_MODE=1 ;
** Running in bug mode: NO **
Highest number: 2
Filtered output ...
2 INT_NET Net : c_c
2 INT_NET Net : b_c
2 INT_NET Net : a_c
** Running in bug mode: YES **
Highest number: 2
Filtered output ...
2 INT_NET Net : c_c
2 INT_NET Net : c_c
2 INT_NET Net : b_c
2 INT_NET Net : c_c
2 INT_NET Net : b_c
2 INT_NET Net : a_c