No, that's all fine. The reason strict doesn't care about this:

my $f = sub { print "a: ", a(), " b: ", b(), "\n";};
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... is because you've used parentheses for the argument lists. This means that the sub calls are not syntactically considered barewords. use strict "subs" only cares about barewords.

I wouldn't necessarily consider it an especially elegant technique, but everything should work. Personally I'd at least try to clean up the subs that add_var installed at the end of mdo:

sub mdo
{
   my @to_be_cleaned;
   while (scalar @_ > 1) {
      my $name = shift;
      my $fn = shift;
      add_var($name,$fn);
      push @to_be_cleaned, $name;
   }
   
   my $fn = shift;
   my $return = $fn->();
      
   require namespace::clean;
   namespace::clean->clean_subroutines('main', @to_be_cleaned);
   
   return $return;
}
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