Another way to make your original code work is to include an empty prototype in the sub definitions. That makes perl take the barewords as functions without argument while compiling. I imagine that is a side effect of constant folding.

my $META_HIDDEN = 1; 
sub META_HIDDEN () : lvalue { $META_HIDDEN }
my $META_EXT    = 'meta'; 
sub META_EXT () : lvalue { $META_EXT }

my $to = $self->abs_loc . 
  ( META_HIDDEN ? "/.$newname." : "/$newname." ) 
  . META_EXT;
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