if (substr($_, 0, 4) eq "sub ")

##</code><code>##

for ($intree->{contents})

##</code><code>##

for my $contents ($intree->{contents})

##</code><code>##

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

##</code><code>##

sub func { ... }
my $ref = \&func;

##</code><code>##

my $ref = sub { ... };