Well, I tried literally everything, including complete de- and re-installation of perl (and "rm -rf" of leftovers) but it did not help.

Looks like a bug?

No. Looks like user error.

Try this, get https://cpan.metacpan.org/authors/id/A/AN/ANDK/CPAN-2.05.tar.gz, untar it and install it

Then try running ... and if it fails post the full error message including output of perl -V

[rum-mad-adtsrv-01] # perl -v

This is perl 5, version 20, subversion 1 (v5.20.1) built for x86_64-li
+nux

Copyright 1987-2014, 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.


[rum-mad-adtsrv-01] # ./cpan.pl
Can't locate package CPAN::Debug for @CPAN::Shell::ISA at ./cpan.pl li
+ne 7.
Can't call method "exists" on an undefined value at /usr/lib/perl5/5.2
+0.1/CPAN/Shell.pm line 1471.
[download]



test script is:



#!/usr/bin/perl

use strict;
use warnings;

use CPAN::Shell;

CPAN::Shell->install('Image::Size');
[download]

Sapienti sat.

CPAN::Debug is part of the CPAN distribution and also part of the Perl 5.20 distribution. So this means that your installed Perl 5.20 is misconfigured or was an incomplete download if it can't find CPAN::Debug.

How did you install Perl 5.20 and/or CPAN?

Also, why did you install it over the system Perl, which belongs to your OS vendor?