Having worked on a lot of code that did such things, I found that I had much fewer regrets with the almost subtly different:

my $msg_body_1 = join '',
   "line 1\n",
   "line 2\n",
   "line3a  \t  \line3b\n",
   "line4",
   "\n\n",
;
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'.' binds tighter than a lot of things that are pretty common to include in a list of things to be joined together. The "no trailing '.' allowed" problem (an endless source of pain for me outside of Perl but in the form of "no trailing comma allowed") goes away when using comma.