Not a HASH reference at /usr/local/share/perl/5.10.0/Algorithm/Huffman.pm line 198.

sub new {
   my ($proto, $key, $value) = @_;
   my $class = ref($proto) || $proto;

   my $self = $class->SUPER::new;

   $self->{"KeyValuePair::key"}   = $key;         # строка 198
   $self->{"KeyValuePair::value"} = $value;
   
   return $self;
}