From my reading of the OP's original post, he wanted a relative program and module structure. This works for an A sub dir holding the program Foo-use, and a parallel level module dir B, holding a module Foo1.pm. The relative nature of the directories means they can be installed anywhere together, and the module in B will be found.

Make 2 subdirs anywhere in an arbitrary directory, named A and B

In A, the program:

#!/usr/bin/perl
use lib "../B";
use Foo1 foo;
use Tk;  # put in to show that @inc is still working fine

foo();
print "Goodbye\n";
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In the parallel B module directory, the module Foo1.pm

#module Foo1.pm located in B

package Foo1;
require Exporter;
 
@ISA = qw|Exporter|;
@EXPORT_OK = qw|foo|;
 
sub foo
{
   print "Hello World, I'm here\n";
}
1;
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Run it:

$  ./A/Foo-use
 Hello World
 Goodbye
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