As another data point, I get the same on This is perl, v5.8.2 built for MSWin32-x86-multi-thread:

Q:\>perl -w  xml-twig-test.pl
a/a/b
a/a/b

a/a/b
a/a/b

a/a/b
a/a/b/a

a/a/b
a/a/b

a/a
a/a

a/b
a/b

a
a
[download]

Trying to force a copy by adding $str = "$str" . "" or other variations didn't prove fruitful...