smsg: parse_references: micro-optimization

smsg: parse_references: micro-optimization

With Perl 5.10+, we can rely on the defined-or-assignment (//=)
operator to avoid repeatedly rewriting an SV.

This may not provide a measurable difference here, but
it's more consistent with current style where things like
commit a05445fb400108e60ede7d377cf3b26a0392eb24
("config: config_fh_parse: micro-optimize") provide a measurable
improvement.