use strict;
use warnings;   

{
    package MyNumber;
    use overload '0+' => \&numify,
                 '/'  => \&division;
    use Scalar::Util;
    my %numbers;
    
    BEGIN {*MyNumber::__ = \&Scalar::Util::refaddr}
                                                     
    sub DESTROY  {delete $numbers {__ shift}}   
    sub new      {my $f; bless \$f => shift}
    sub set      {$numbers {__ $_ [0]} = $_ [1]; $_ [0]}
    sub numify   {$numbers {__ $_ [0]}}
    sub division {my $f = $numbers {__ $_ [0]};
                  my $s = ref ($_ [1]) =~ /MyNumber/ ? $numbers {__ $_
+ [1]}
                                                     : $_ [1];
                  ($f, $s) = ($s, $f) if $_ [2];
                  $s ? $f / $s : undef}
}

my $fig_1  = MyNumber -> new -> set (get_numeric_value_from_xml (...))
+;
my $fig_2  = MyNumber -> new -> set (get_numeric_value_from_xml (...))
+;
my $growth = 100 * $fig_1 / $fig_2 - 100;
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