my @functions = (
                 sub { # sin(x)
                    my $in = shift;
                    return sin($in);
                  },
                 sub { # add(x,y)
                    my ($left,$right) = @_;
                    return ($left + $right);
                  },
                 sub { # mul(x,y)
                    my ($left,$right) = @_;
                    return ($left * $right);
                  },
                 sub { # log(x)
                    my $in = shift;
                    return ($in == 0) ? 0 : log(abs($in));
                  },
                );


...

  sub evalTree {
    my $intree = shift;
    my $param = shift;

    # A shortcut for the current node.
    for my $contents ($intree->{contents}) {
      return 0 unless defined $contents;

      if (ref($contents) eq 'CODE') {
        return $contents->(
           evalTree($intree->{left},  $param),
           evalTree($intree->{right}, $param),
        );
      }

      if ($contents =~ /^[A-Za-z]$/) {
        return $param->{$_};
      }

      return $contents;
    }
  }