This is pretty clean, put your private stuff into a sub-package and alias that long package path it to a shorter package name at compile time.

(of course these subs won't be able to call imported subs in the parent package without full qualification or inheritance ;-)

use strict;
use warnings;

package X::Y::Z::my;                    # sub package

sub xyz {warn "xyz called" }


package Y::Y::Z;

BEGIN {
    *::my:: = *X::Y::Z::my:: ;          # alias sub package to my::
}

# END {                                 # delete alias at file's end
#     delete $::{"my::"};
# }



my::xyz();


BEGIN {                                 # delete alias here for demo
    delete $::{"my::"};
}

package Other;

my::xyz();                              # fails alias already gone
[download]

C:/Perl_524/bin\perl.exe  d:/exp/local_package.pl 
xyz called at d:/exp/local_package.pl line 6.
Undefined subroutine &my::xyz called at d:/exp/local_package.pl line 3
+0.
[download]

update

the magic in the BEGIN/END blocks could be done in the importer of a module called with

use  PackageAlias "X::Y:Z::my" => "my" and no PackageAlias "my"

Update

I just realised that Package::Alias already exists, even with a pretty similar interface.

Skipped colons in my example to avoid confusion.