Mark postderef as experimental for 5.20 compatibility

Postderef became canon in Perl 5.24. In 5.20 it was still considered
experimental.

tsplotv

@@ -1,4 +1,4 @@
-#!/usr/bin/perl -w 
+#!/usr/bin/perl
 
 =head1 NAME
 
@@ -39,7 +39,10 @@ other.
 
 =cut
 
-use strict;
+use v5.20;
+use warnings;
+use experimental qw(postderef);
+
 use TimeSeries;
 use Getopt::Long;
 use Pod::Usage;