Like so:

package bar;
use parent 'Modern::Perl';

# rest of bar stuff

1;
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If you need to use import for something else in bar.pm, you'd need:

package bar;
use Modern::Perl;

sub import {
    ...
    goto &Modern::Perl::import;
}
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(Or you could just copy Modern::Perl's source as bar.pm and add to it...)