I get the same result on Solaris 9. I'm a little baffled by it, too, because at line 10, right before the print statement, $period has this structure:

  DB<1> x $period
0  Date::Set=HASH(0x5ef384)
   'dtstart' => undef
   'fixtype' => 1
   'list' => ARRAY(0x1131ac)
      0  HASH(0x606130)
         'a' => Date::Set::ICal=HASH(0x605d94)
            'epoch' => 20021201
         'b' => Date::Set::ICal=HASH(0x497a74)
            'epoch' => 20021231
         'open_begin' => 0
         'open_end' => 0
   'tolerance' => 0
   'type' => 'Date::Set::ICal'
   'wkst' => 'MO'
[download]

But if I print $period, I get

  DB<2> p $period
[19700820T172641Z..19700820T172711Z]
[download]

I suggest contacting the developer(s). I also noticed that one of the dependent modules warned about the API being in flux, so there is a possibility that changes in the API broke Date::Set. You might want to try older versions of those.

Good luck.