{
my ( $test, $modify );
sub walk {
     my $ref;
     ( $test, $modify, $ref ) = @_;
     ref $modify eq 'CODE' and ref $test eq 'CODE' or croak 'First 2 arguments must be coderefs';
     return _walk($ref);
}

sub _walk {
     my ( $ref ) = @_;
     return $modify->($ref) if $test->($ref);
     given ( ref $ref ) {
          when ( '' ) { return $ref; }
          when ( 'ARRAY' ) { return map { _walk($_) } @$ref; }
          when ( 'HASH' ) { return map { $_ => _walk($ref->{$_}) } keys %$ref; }
          default { carp "I'm confused by $ref" }
     }
}
}