Answering my own question, $t was still in the pad of the calling scope in my example. ¹

Otherwise the problem persists:

 
use v5.12;
use warnings;
say $];



{
  my $t = 42;

  sub foo { say eval $_[0] }
}

foo('$t');


-UUU:----F1  t_dyna_scope.pl   All L13    (CPerl) ---
-*- mode: compilation; default-directory: "~/perl/" \
-*-
Compilation started at Fri Aug 30 11:25:02

perl t_dyna_scope.pl
5.036000
Variable "$t" is not available at (eval 1) line 1.
Use of uninitialized value in say at t_dyna_scope.pl\
 line 10.


Compilation finished at Fri Aug 30 11:25:02
[download]

Update

¹) interestingly a way to access private vars in the dynamic scope ... Quite surprising

Update

It's not dynamic scope either, probably only file scope... Needs further testing