If you are wanting to add a directory to the include paths for modules you can:

use lib qw(/your/path);
use YourModule;
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The lib pragma will basically add that directory to @INC so you can include modules from that directory. If you want to set that env variable from within the script you can:

BEGIN {
    $ENV{PERL5LIB} = "/foo";
}
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Cheers,
KM