For example, suppose you have your own directory under /home/gilligan/lib, and you place your own Navigation::SeatOfPants module in /home/gilligan/lib/Navigation/SeatOfPants.pm. Simply saying:

use Navigation::SeatOfPants;

is unlikely to do anything useful because only the system directories (and typically the current directory) are considered for @INC. However, even adding:

push @INC, "/home/gilligan/lib";   # broken
use Navigation::SeatOfPants;

doesn't work. Why? Because the push happens at runtime, long after the use was attempted at compile time. One way to fix this is to add a BEGIN block around the push:

BEGIN { push @INC, "/home/gilligan/lib"; }
use Navigation::SeatOfPants;

However, this is noisy and prone to require far more explanation than you might be comfortable with, especially for the maintenance programmer who has to edit your code later. Let's replace all that clutter with a simple pragma:

use lib "/home/gilligan/lib";
use Navigation::SeatOfPants;

Because a use lib pragma will pretty much always have a site-dependent pathname, it is traditional and encouraged to put it near the top of the file. This makes it easier to find and update when the file needs to move to a new system or when the lib directory's name changes. (Of course, you can eliminate use lib entirely if you can install your modules in a standard @INC locations, but that's not always practical.)

Think of use lib as not "use this library," but rather "use this path to find my libraries (and modules)." Too often, you see code written like:

use lib "/home/gilligan/lib/Navigation/SeatOfPants.pm"; # WRONG

and then the programmer wonders why it didn't pull in the definitions. Be aware that use lib indeed runs at compile time, so this also doesn't work:

my $LIB_DIR = "/home/gilligan/lib";
...
use lib $LIB_DIR;     # BROKEN
use Navigation::SeatOfPants;

use constant LIB_DIR => "/home/gilligan/lib";
...
use lib LIB_DIR;
use Navigation::SeatOfPants;