Similarly to system, if the string contains no shell metacharacters then it will executed directly.
Not sure what is included into "shell metacharacters" but a simple semicolon seems to suffice to force execution via the shell ...
lanx@ubuntu14-large:/tmp/sym$ perl -MCwd -E'say `pwd;` ' /tmp/sym lanx@ubuntu14-large:/tmp/sym$ perl -MCwd -E'say `pwd` ' /tmp/dir lanx@ubuntu14-large:/tmp/sym$
(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery