If you want application-specific modules to be relative to the application's main script, combine use lib with use FindBin to get something like the following:

use FindBin;
use lib "$FindBin::Bin/lib";

use WidgetCorp::Widget;
use WidgetCorp::Mangler;
use Text::Distort;
use Wiggle;
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Now you can structure your application's private modules thusly.

./widgettool.pl
./lib/Wiggle.pm
./lib/Text/Distort.pm
./lib/WidgetCorp/Widget.pm
./lib/WidgetCorp/WidgetMangler.pm
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I emphasize that this is useful for application-specific or highly customized modules. For standard stock or CPAN modules, though, it's still preferable to have these installed properly for all users of the system.