Having the perldoc directly in another Emacs pane without switching the application has many advantages:

• you can edit the file directly, for instance search and cut and paste code examples from a module
• I can build keyboard-macros which use this text
• you get an index inside imenu to navigate it
• this imenu index is shown in a left pane for navigation when ECB or similar is activated
• auto-expansion like dabbrev-expand is parsing the man buffer for possible expansions
• revisiting the man-page is faster if the man-buffer already exists inside emacs
• the internal window-management sparse me from adjusting the browser and editor window
• ... (certainly forgot to mention other stuff)

Additionally does cperl-mode come with more DWIM logic, like cperl-perldoc-at-point which reads the word under the cursor and automatically calls perldoc -f <word> instead of perldoc <word> if it's a function.