open FILE, ">> $path_to_file" or die "can't open $path_to_file: $!"; 
print FILE "new line\n"; 
close FILE;

{ 
  local $^I = ".bak";  # the backup extension 
  local @ARGV = $path_to_file;  # the file to edit 
  while (<>) { 
    next if $. == 10;  # this will skip line 10 
    print "before line 3!\n" if $. == 3;  # this appears before line 3 
    print;  # this prints the line itself 
    print "after line 6!\n" if $. == 6;  # this appears after line 6 
  } 
}

{ 
  local ($^I, @ARGV) = (".bak", @list_of_files); 
  while (<>) { 
    print unless /^#/; 
  } 
}