#!/usr/bin/perl -w
use strict;

use constant DEBUG => 1; 

my @data = sort <DATA>; # bring all data into memory and sort
                        # other techniques are possible if this
                        # file is so big that this is not feasible

my $current_first_letter = '';

foreach my $line (@data)
{
   print "Processing $line" if DEBUG;
   
   #
   # skip entries not beginning with lower case a-z
   #
   
   my $this_first_letter;
   next unless ( ($this_first_letter) = $line =~ /^([a-z])/ ); 
   
   #
   # Start a new file when we change first letters
   #
   
   if ($this_first_letter ne $current_first_letter)
   { 
      my $filename = $this_first_letter . "_words.txt";
      print "Creating New File...$filename...\n" if DEBUG;  
      
      #this over-writes any existing file of the same name
      #
      open FILE, '>', $filename or die "can not open $filename $!"; 
      $current_first_letter = $this_first_letter;
   }
   
   #
   # write to the current file
   #
   print "....adding $line";
   print FILE $line;   
}

=prints -- look in the output files for the rest..
        -- set DEBUG =>0 to supress this ouput

Processing Bob/person
Processing d/some fictional animal
Creating New File...d_words.txt...
....adding d/some fictional animal  #a sort test case #
Processing dog/pet 
....adding dog/pet 
Processing doggie/also a pet
....adding doggie/also a pet
Processing horse/an animal
Creating New File...h_words.txt...
....adding horse/an animal
=cut

__DATA__
dog/pet 
d/some fictional animal
horse/an animal
doggie/also a pet
Bob/person