The docs suggest:

 my $folder = $mgr->open('=in/new');
 my @subs = $folder->listSubFolders;

 my @subs = Mail::Box::IMAP4->listSubFolders(folder => '=in/new');
 my @subs = Mail::Box::IMAP4->listSubFolders; # toplevel folders.
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But if you are using local folders, don't you want Mail::Box::Mbox? (NB, the docs indicate that mbox files "For MBOX folders, sub-folders are simulated."