$ cat lib/Foo/Bar.pm 
package Foo::Bar;
use warnings;
use strict;
use Cwd qw/abs_path/;

sub foo {
	my $callerfile = (caller)[1];
	print "DEBUG: caller=<$callerfile> \$0=<$0>\n";
	return abs_path($callerfile);
}

1;

$ cat y.pl 
use warnings;
use strict;
use Foo::Bar;
print Foo::Bar::foo(), "\n";

$ cat x.pl 
use warnings;
use strict;
do './y.pl';

$ perl -Ilib x.pl 
DEBUG: caller=<./y.pl> $0=<x.pl>
/tmp/y.pl