Before. I took the approach of identifying difference with what I would do. I immediately saw two things: code before functions, and use of exit. I looked if there was more code below exit, and voila!

The fix is to add one word:

#!/usr/bin/perl

use strict;
use warnings;

use Data::Dumper;

_test();
exit;

######## subroutines

BEGIN {                           <-----
   my @values = qw( 1 2 3 );

   sub _test {
      print "Values:\n", Dumper( \@values );
   }
}
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