Perl undefined variables warnings when tyring to inherit from a non existing thorn

Erik Schnetter reported warnings from Perl of the sort:

Use of uninitialized value in concatenation (.) or string at /Users/eschnett/src/CarpetX/Cactus/repos/flesh/lib/sbin/interface_parser.pl line 176.

on Gitter. These seem triggered by trying to inherit from a non-existing thorn

CST error in /Users/eschnett/src/CarpetX/Cactus/repos/flesh/lib/sbin/interface_parser.pl (at 296)
  -> IDBRILLDATA (thorn IDBrillData) inherits from ellbase
    No thorn in your current ThornList implements ellbase
    Either remove IDBrillData, or add a thorn to your
     ThornList implementing ellbase
    No thorns in arrangements directory implement ellbase

and Perl trying to use a not yet defined array hash value when constructing the list of thorn ancestors.

There are no such warnings when the code builds, yet the Perl code should not produce these warning even in the case of a later build failure.

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