[SPOILER] Perl 'Easy' Quiz of the Week #2005-1

Huh.  No one so far seems to have hit upon the solution I hit upon, which 
was:  (I've deliberately left out most error checking since I want the 
main script body uncluttered)

#!/usr/bin/perl
use strict;

open(STDIN, '<', shift) if @ARGV;
open(STDOUT, '>', shift) if @ARGV;

my $mline='';
my $prefix='';

while(<>) {
  /^(\w+)\.[A-Z]$/ or die "Bad line: $_";
  if ($prefix ne $1) {
    $prefix = $1;
    print $mline;
    $mline = "$prefix.M\n";
  }
  if ($mline lt $_) {
    print $mline;
    $mline = '';
  }
  print;
}
print $mline;
__END__


If I were into justifying my code with grand principles, I'd point out 
that this is an example of the "null object" pattern 
(http://c2.com/cgi/wiki?NullObject) - I end up doing {print $mline} often, 
but when $mline is the empty string, this print statement has no effect. 
Therefore I don't need several extra variables to track whether I've 
already printed the proper thing or not, or whether I'm dealing with the 
first line of the file, etc.  I dislike special cases, because I always 
get into 1-off errors when dealing with them.  Using {print ''} as a no-op 
allows me to do away with special cases.