Using Algorithm::Loops::NestedLoop:

use Algorithm::Loops qw( NestedLoops );

sub expand {
   my $r = (@_ ? $_[0] : $_);
   my @ends = (split(/-/, $r), $r);
   return [ $ends[0] .. $ends[1] ];
}

while (<DATA>) {
   s/#.*//;
   s/\s+\z//;

   my @ranges = map expand, split /\./, $_;
   my @ips = NestedLoops(
      \@ranges,
      sub { join('.', @_) },
   );

   foreach my $ip (@ips) {
      print("$ip\n");
   }
}

__DATA__
172.17.119.2 # Comments are here...
172.17.119.4-5 # Comments are here...
172.19-21.254.2-3 # Comments are here...
192.168.1.1-3 # Comments are here...
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The above can also be written as an iterator:

while (<DATA>) {
   s/#.*//;
   s/\s+\z//;

   my @ranges = map expand, split /\./, $_;
   my $iter = NestedLoops(\@ranges);

   while (my @parts = $iter->()) {
      my $ip = join('.', @parts);
      print("$ip\n");
   }
}
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Update: Factored out expand. It makes it much easier to read.