opendir requires closedir (not close).

A common technique when using sort is to precalculate anything expensive (-M in this case) using a map before the sort then extract the desired data using another map after the sort:

my @list = map {$_->[1]}
    sort { $a->[0] <=> $b->[0] }
    map {[-M "$maillog_dir/$_", $_]}
    ...
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