You would need the two methods to be defined something like:

sub FIRSTNODE
{
    my ($root_node) = @_;

    my $node = $root_node;

    while ($node->{left})
    {
         $node = $node->{left};
    }

    return $node;
}

sub NEXTNODE
{
     my ($node) = @_;

     if ($node->{right})
     {
          $node = FIRSTNODE($node->{right});
     }
     elsif ($node->{parent})
     {
          if ($node->{parent}->{left} == $node)
          {
                $node = $node->{parent};
          }
          else
          {
                while ($node->{parent}->{right} == $node)
                {
                      $node = $node->{parent};
                }

                return unless $node;

                $node = $node->{parent};
          }
     }
     else
     {
          $node = undef;
     }

     return $node;
}
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The NEXTNODE method there traverses the tree, looking for the next logical node. This assumes that your B-Tree implementation uses a hash-style tree with references between nodes. To switch it to using text keys instead, you could write wrapper methods, as proposed below, or modify the code slightly to set $key instead of $node.

The FIRSTKEY/NEXTKEY methods would merely return the key instead of the node reference, something like:

sub FIRSTKEY
{
     my $node = FIRSTNODE($root_node);

     return $node && $node->{key};
}

sub NEXTKEY
{
     my ($key) = @_;

     my $node = NEXTNODE (FindNode ($key));

     return $node && $node->{key};
}
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