open (FILE, "<$file") ;

my %word_counter;

while (<FILE>)  {
  my @words_line = split(/\s/, $_) ;
  map { $word_counter{$_}++ } @words_line ;
}

my @sorted_keys = sort { $word_counter{$b} <=> $word_counter{$a}  } keys %word_counter ;

#my @sorted_keys = map  { $_->[0] }
          #sort { $b->[1] <=> $a->[1] }
          #map  { [$_, $word_counter{$_}] } keys %word_counter ;