nothing.

PERLDOCDEBUG=1; export PERLDOCDEBUG;


 perldoc -F ./perlmonks.pl
'opt_o_with: Noting "text" as desired output format...
'opt_o_with: Considering Pod::Perldoc::Totext
'opt_o_with: Considering Pod::Perldoc::Totext
'opt_o_with: Considering Pod::Perldoc::ToText
'opt_o_with: Considering Pod::Perldoc::ToTEXT
'opt_o_with: Considering Pod::Simple::text
'opt_o_with: Considering Pod::Simple::text
'opt_o_with: Considering Pod::Simple::Text
'opt_o_with: Considering Pod::Simple::TEXT
'opt_o_with: Considering Pod::text
'opt_o_with: Considering Pod::text
'opt_o_with: Considering Pod::Text
'opt_o_with: Considering Pod::TEXT
'opt_o_with: Noting "term" as desired output format...
'opt_o_with: Considering Pod::Perldoc::Toterm
'opt_o_with: Considering Pod::Perldoc::Toterm
'opt_o_with: Considering Pod::Perldoc::ToTerm
'opt_o_with: Considering Pod::Perldoc::ToTERM
'opt_o_with: Considering Pod::Simple::term
'opt_o_with: Considering Pod::Simple::term
'opt_o_with: Considering Pod::Simple::Term
'opt_o_with: Considering Pod::Simple::TERM
'opt_o_with: Considering Pod::term
'opt_o_with: Considering Pod::term
'opt_o_with: Considering Pod::Term
'opt_o_with: Considering Pod::TERM
'GetOptsOO::getopts: Starting switch processing.  Scanning arguments [
+-F ./perlmonks.pl]
'GetOptsOO::getopts: Ending switch processing.  Args are [./perlmonks.
+pl] with 0 errors.
'pagers_guessing: Pagers: /usr/bin/sensible-pagermorelesspgviewcat'pro
+cess: /usr/bin/perldoc => Pod::Perldoc v3.2503
Attempting to drop privs...
OK, I dropped privileges.
'process:
'find_good_formatter_class: Formatter class Pod::Perldoc::ToTerm versi
+on 3.25 successfully loaded!
'find_good_formatter_class: Will format with the class Pod::Perldoc::T
+oTerm
'grand_search_init: Searching for ./perlmonks.pl
[download]

In my old debian 4 (etch) I've look to ENV, but nothing for PERLDOC is set, and I'm nothing set after install my new system (debian 9.4 stretch).
I've only install perdoc on debian 9.4 from mirror, not from CPAN.

The perl versin on stretch:

  perl -v

This is perl 5, version 24, subversion 1 (v5.24.1) built for x86_64-li
+nux-gnu-thread-multi
(with 75 registered patches, see perl -V for more detail)

Copyright 1987-2017, Larry Wall

Perl may be copied only under the terms of either the Artistic License
+ or the
GNU General Public License, which may be found in the Perl 5 source ki
+t.

Complete documentation for Perl, including FAQ lists, should be found 
+on
this system using "man perl" or "perldoc perl".  If you have access to
+ the
Internet, point your browser at http://www.perl.org/, the Perl Home Pa
+ge.
[download]

regrads Henrik