DB<135> sub tst { my $doc="bla" }
 => 0
 
  DB<136> PadWalker::peek_sub(\&tst)
 => { "\$doc" => \undef }
 
  DB<137> tst()
 => "bla"
 
  DB<138> PadWalker::peek_sub(\&tst)
 => { "\$doc" => \undef }