Like Mr. Muskrat mentioned above, the documentation for glob mentions:

In list context, returns a (possibly empty) list of filename expansions on the value of EXPR such as the standard Unix shell /bin/csh would do

So one needs to look to find what the documentation for /bin/csh says. If you search around the web for csh documentation who will come across a section on "filename substitution" (aka globbing). In short what this documentation says is that if it contains a "*", "?","[", or "~" then it an error not to have any files match. On the other hand it mentions nothing of "{".

It also mentions that something like "a{b,c,d}e" is metanotation for a shorthand way of writing "abe ace ade" which is the trick played above with the glob function to get a list of possible combinations of card values and suits, i.e.,:

my @deck = glob "{2,3,4,5,6,7,8,9,T,J,K,Q,A}{C,D,H,S}";
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HTH

-enlil