Like this:

# in file Module1.pm
package Module1;
use Module2;

# in file Module2.pm
package Module2;
use Module1;
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Perl will remember which modules are already loaded (in the %INC hash) and everything sorts itself out in the end™.

-- 
Joost       downtime n. The period during which a system
            is error-free and immune from user input.
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