glob does more or less the same thing as opendir /readdir, except that it is more practical: first, it's one line instead or two, second it removes pseudo-directories such as "." and ".." which you usually don't want to explore, and finally it returns a list of files with their path, so that you can open them without having to add the path.

unless does exactly what the English meaning of the word implies. It is essentially equivalent to if not. For example:

my $q = $c / $d unless $d == 0; # avoid division by zero error
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