The comments of almut and other(s) along the lines of "But why...?" should be heeded.
But if you want to press on regardless, here's a slightly different approach:

>perl -wMstrict -le
"package BlaBal;
 sub hi { print 'hi there ', $_[0] };
 sub new {
   my $class = shift;
   return bless { test123 => \&hi } => $class;
   }
 package main;
 my $obj = BlaBal->new;
 $obj->{ 'test' . (120+3) }->('sailor');
"
hi there sailor
[download]

BTW: The statement
    return bless {}, __PACKAGE__;
in the code in Re^4: concat method inside method call does what bless does by default. So the statement
    return bless {};
does the same and is 'prettier'! (However, a constructor so defined cannot be used by derived classes, so this is perhaps a bug, or at least a mis-feature.)