To answer the "root" question.

Looks like the redesign of perldoc.perl.org changed the anchors considerably: *

<dt>30</dt><dd><a id="open-FILEHANDLE%2cEXPR"></a><strong>open FILEHAN
+DLE,EXPR
   </strong>
</dd>
<dt>30</dt><dd><a id="open-FILEHANDLE%2cMODE%2cEXPR"></a><strong>open 
+FILEHANDLE,MODE,EXPR</strong>
</dd>
<dt>30</dt><dd><a id="open-FILEHANDLE%2cMODE%2cEXPR%2cLIST"></a><stron
+g>open FILEHANDLE,MODE,EXPR,LIST</strong>
</dd>
<dt>30</dt><dd><a id="open-FILEHANDLE%2cMODE%2cREFERENCE"></a><strong>
+open FILEHANDLE,MODE,REFERENCE</strong>
</dd>
<dt>30</dt><dd><a id="open-FILEHANDLE"></a><strong>open FILEHANDLE</st
+rong>
[download]

so you'll need  [doc://perlfunc#open-FILEHANDLE] -> open FILEHANDLE now.

not sure how older links in the monastery are effected.*

anyway use [doc://open], this always works.

Update

*) couldn't find any evidence for this, older posts with such links seem to work flawlessly

see https://perlmonks.org/?node_id=3989;BIT=%5Bdoc%3A%2F%2Fperlfunc%23