Try this ...

my %dictionary;

readWords(\%dictionary);
sortDictionary(\%dictionary);

sub readWords {
   $numWords = 0;

   open FILE, $ARGV[0] or die $!;

   while (my $lines = <FILE>) {
      $numWords++;
      chomp $lines;

      my ($words, $definitions) = split('/', $lines);
      $words = lc($words);
      $dictionary{$words} = $definitions;
   }
   foreach $words (keys(%dictionary)) {
      print "$words = ",$dictionary{$words},"\n";
   }
   close FILE;
}

sub sortDictionary {
   foreach my $key (sort keys %dictionary) {
      $key =~ /^(.)/;
      my $line = $key . '/' . $dictionary{$key} . "\n";
      print $line;
   }
}
[download]

Hope that is what you expected ...

C:\> x.pl x.txt
cat = nice animal
snake = nasty animal
dog = best friend
cat/nice animal
dog/best friend
snake/nasty animal
[download]