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my $x1 = Foo::Number->new()->value( 10 );
my $x2 = Foo::Number->new()->value( 20 );
...

my $c1 = $v1 - $x1 + $x2 + $x3 - $x4;
my $c2 = $c1 * $v1;

my $c3 = ( $x1 + $x2 ) * $v1;

use overload 
  '(' => \&left_parenthesis,    
  ')' => \&right_parenthesis,