Set the environment variable PERL5LIB or PERLLIB for all the scripts

export PERL5LIB=/path/to/lib

Adding a path to the beginning of @INC. Good for the specific script

BEGIN {
   unshift @INC, '/path/to/lib';
}

The same but with the standard tool:

use lib '/path/to/lib';           # good for the specific script

On the command line. Good for this invocation only.

perl -I /path/to/lib script.pl