perl: Fix binary detection (#14536)

It seems that stable versions of perl also install a `perlX.Y.Z` binary.
However, it seems that this binary can hang if used in conjunction with
Spack's sbang workaround, as observed during automake's build.

var/spack/repos/builtin/packages/perl/package.py

@@ -305,7 +305,7 @@ def command(self):
         Returns:
             Executable: the Perl command
         """
-        for ver in (self.spec.version, ''):
+        for ver in ('', self.spec.version):
             path = os.path.join(self.prefix.bin, '{0}{1}'.format(
                 self.spec.name, ver))
             if os.path.exists(path):