> Are there any limitations, gotchas, or risks to using this syntax? (And, if so, what are they?)

yes

  DB<109> @A = qw/{a,c}{b,d} x/;
 => ("{a,c}{b,d}", "x")

  DB<110> print "$x\n" while ($x = <@A>)
ab
ad
cb
cd
x
[download]

> Is this documented anywhere?

yes

       glob EXPR
       glob    In list context, returns a (possibly empty) list of filename
               expansions on the value of EXPR such as the standard Unix shell
               bincsh would do. In scalar context, glob iterates through
               such filename expansions, returning undef when the list is
               exhausted. This is the internal function implementing the
               "<*.c>" operator, but you can use it directly. If EXPR is
               omitted, $_ is used.  The "<*.c>" operator is discussed in more
               detail in "I/O Operators" in perlop.

> Has anyone ever used this syntax in their scripts before?

yes --> Re: Substitution of a particular letter in all combinations of positions in word (see inside)