# set up to iterate over the *.txt files in the current directory,
# editing in place and saving the old file with a .orig extension
local $^I   = '.orig';              # emulate  -i.orig
local @ARGV = glob("*.txt");        # initialize list of files
my $fileindex=0;
my $hostname;
while (<>) {
  $hostname=$1
    if (/Hostname:\s(\S+)/);

  print "# $hostname\n"
    if (/:$/);
  print;
} continue {
  if(eof){
    close ARGV;
    rename $ARGV[$fileindex], "${hostname}.txt"
      if ($hostname); # Careful, this will overwrite files.
    $fileindex++;
    $hostname=undef;
  }
}