One more way to do it: 
package my::foo;
BEGIN { $INC{'my/foo.pm'} ||= 'dev/null'; }
require exporter; # etc.
package main;
use my::foo;

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The addition to %INC tells use/require that the package is already in memory, so it won't try to load it again.
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In reply to Re^3: multiple "sub" packages in 1 file: possible? how? by Beechbone
in thread multiple "sub" packages in 1 file: possible? how? by perl-diddler

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