The easy answer is:

use lib qw(/my/path/to/add/);
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This adds directories to @INC at compile-time, so they're valid in subsequent use statements. Try perldoc lib for more information on this.

The longer answer:

You could do this to add things to @INC:

push @INC, '/my/path/to/add';
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because, @INC is just a Perl array, nothing special. However, use statements are executed at compile time, not at runtime, so if you simply push onto the array, @INC won't be changed until runtime, and your use still won't find what it's looking for.

Pushing things directly onto @INC would work if you're just doing require, which happens at runtime. But the short answer is best: just  use lib and you'll be happy.

Alan