But abs_path is taking care of resolving symlinks, isn't it?

Yes, but so is FindBin, with more effort and less code in your script - you can have a look at the source. Note it's been a core module for a long time.

So is there still an issue?

Not necessarily, if you are sure you'll always be running this script on this OS. But if you want portability, then you need to use the appropriate modules to ensure that, i.e. File::Spec or Path::Class. And personally I wouldn't use .. to remove the script's filename from the path (another advantage of $FindBin::Bin).

# add a "lib" that is in the script's dir
use File::Spec::Functions qw/catdir/;
use lib catdir($FindBin::RealBin, 'lib');

# add a "lib" that is in the script's parent dir
use Path::Class qw/dir/;
use lib dir($FindBin::RealBin)->parent->subdir('lib')->stringify;
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You might also be interested in File::FindLib (although because that does a search, it is less deterministic).

Thanks again!

So am I right assuming that this should be the most portable way when I want to use just core modules?

use File::Basename;
use File::Spec::Functions;
use lib catdir( dirname( $FindBin::RealBin ), 'lib' );
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---

s$$([},&%#}/&/]+}%&{})*;#$&&s&&$^X.($'^"%]=\&(|?*{%
+.+=%;.#_}\&"^"-+%*).}%:##%}={~=~:.")&e&&s""`$''`"e

The File::Basename documentation says:

dirname() and basename() emulate the behaviours, and quirks, of the shell and C functions of the same name. ... dirname ... This function is provided for compatibility with the Unix shell command dirname(1) and has inherited some of its quirks. In spite of its name it does NOT always return the directory name as you might expect.

Although it doesn't say what exactly those quirks are. To get a directory ../lib relative to the script into @INC using only core modules, I might suggest the following, although there may be some tricky symlink situations where they return different results (abs_path goes out to the filesystem, the second code example does not). Then the question becomes how likely you think that might be. Test cases are best :-)

use FindBin;
use File::Spec::Functions qw/updir catdir/;
use Cwd qw/abs_path/;
use lib abs_path( catdir( $FindBin::RealBin, updir, 'lib' ) );

# - or -

use FindBin;
use File::Spec::Functions qw/splitdir catdir/;
use lib do { my @p = splitdir $FindBin::RealBin;
    catdir @p[0 .. $#p-1], 'lib' };
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