You need to prototype your sub if you want to use it like a builtin (see perlsub) inside of the sub definition.

sub walk($);

sub walk($) {
  my $t = shift();
...
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Either that or put parens around the argument in the sub.

  ...
  else {
    walk( $val );
  }
  ...
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I guess the problem is that, inside of the sub definition, walk is not yet defined.