my $fil = shift;
open (FIL, $fil) || die ("can't read $fil: $!");
my $filstr = do { local $/; <FIL> };
close FIL;

my %charhash;
$charhash{$_}++ for ( split //, $filstr );

my $ent = entropy(\%charhash, length( $filstr ));

printf ("file %s\ncontents     entropy = %30.20f\n",$fil,$ent);


sub entropy {
  my ($hashref, $total, $baselog) = @_;
  $baselog = 0.693147180559945 unless $baselog; # log(2)
  return undef unless ( ref $hashref and $total > 0 );
  my $sum;
  $sum += $_ * (log($_)/$baselog) for ( map { $_/$total } values %$hashref );
  return -$sum;  
}