You need to identify the lines without * to let those pass through. Then loop over the network list but use a copy of the line in the substitution each time

#!perl
use strict;
my @finalnwlist = qw(
  192.169.32.0/255.255.255.0
  192.169.72.0/255.255.255.0
  192.169.73.0/255.255.255.0);

replace_exports('exports.txt');
          
sub replace_exports {

  my $filename = shift;
  open IN,'<',$filename 
       or die "Error opening $filename for read : $!";
  my @lines = <IN>;
  close IN;

  open OUT, '>', 'chg_'.$filename
       or die "Error opening $filename for write : $!";
       
  for (@lines) {
    chomp;
    if (m/\*/){
      print "Change $_\n";
      for my $nw (@finalnwlist){
        my $copy = $_;
        $copy =~ s/\*/$nw/;
        print OUT $copy."\n";
      }
    } else {
      print OUT $_."\n";
    }
  }
  close OUT
}
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