In your initial example, I could see some merit in doing:

my $BigCalc = do {
    my $SmallCalc;
....
    $smallCalc;
};
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But for the most part, the only occasions I've felt the need to use bare-blocks are:

• To limit the scope of locks on shared vars:

  { lock $fred; $fred = 3; }
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• For limiting the scope of closures:

  {
      my $var;
      sub fred {
          if( $var++ ) {
               ...;
          }
          ...
      }
  }
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  This use has mostly been superceded by state variables.

In neither case have I felt the need to introduce the opening brace in any way. Indeed, I think to do so would be more rather than less likely confusing; potentially flattering to deceive that the use of the construct was more important than it is.