Find out where FooPackageName was loaded from, and then use this information as a base for finding your text files:

    use Foo::Foo1::Foo2::FooPackageName;
    my $file = "file.txt";
    my $path = $INC{"Foo/Foo1/Foo2/FooPackageName.pm"};
    $path =~ s/FooPackageName.pm/$file/;
    print "Full path to $file: $path \n";
[download]

(I use this inside the Inline::Parrot module, in order to find out where the "pir" files were installed)