It looks like you are missing the path to your library in your @INC array within Perl. You can add a line to your script to explicitly use the library directory where Image::Grab is installed.

use lib '/var/local/perl-site/';
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Or where ever it may be. It is strange that it is not in a standard system patch though if it is installed with cpan. Do you know the directory where Grab.pm is installed?

Cheers,
R.