Run /usr/bin/perl directly, rather than using the #! mechanism.

When TMPDIR is on a filesystem mounted with the noexec flag, then you can't just run the downloaded perlbrew.  This patch makes the install script run perl with the downloaded perlbrew as its first argument.

Using the hardcoded '/usr/bin/perl' seems wrong to me, but perlbrew has the #! line hardcoded to the same thing, so I guess it's ok?