Yes, it is a builtin, but:

10] perldoc -f glob
       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 /bin/csh 
               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.

               Beginning with v5.6.0, this operator is 
               implemented using the standard "File::Glob" 
               extension.  See File::Glob for details.
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So the builtin glob calls load the File::Glob module on the fly apparently