You're trying to link to the length function at perldoc.com. That site is tremendously unstable these days, which is why the Monsatery's POD linking mechanism defaults to perldoc.perl.org. Using square brackets you can link to the pod like this: [doc://length] and get this result: length. Do be sure to spell it right when you link.

if (length($var) eq 58) {
#do something
}
[download]

...should use ==, not eq since you're doing a numeric comparison.