perl6 error string is injected into path var

Description of issue

ftplugin/perl6.vim executes a shell command when it's invoked without checking v:shell_error:

            if &shellxquote != '"'
                let perlpath = system('perl6 -e  "@*INC.join(q/,/).say"')
            else
                let perlpath = system("perl6 -e  '@*INC.join(q/,/).say'")
            endif

Whatever this was originally for, it's not valid code in current versions of rakudo:

$ perl6 -e '@*INC.join(q/,/).say'
Dynamic variable @*INC not found
  in block <unit> at -e line 1

...and this error text ends up in the value of :set path. I'm not sure what the original intent of this code was, but the thought that it's been executing random commands every time I open the editor on a perl6 file is a bit scary.

Output of vim --version

VIM - Vi IMproved 8.0 (2016 Sep 12, compiled Apr  1 2018 21:14:00)
Included patches: 1-1645
Modified by Gentoo-8.0.1645

Output of perl6 -v

This is Rakudo version 2018.03-163-g7faf431fac2a built on MoarVM version 2018.03-89-gefa8e16b91d4
implementing Perl 6.c.