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use warnings;
use strict;
...
print Dumper BB => $b1 - $a0; 
print Dumper CC => $b1 - $b0;
print Dumper DD => $a1 - $b0;

>perldoc -f can
No documentation for perl function 'can' found

    This feature is available from Perl 5.31.6 onwards when enabled by
+ "use
    feature 'isa'". This feature is enabled automatically by a "use v5
+.36"
    (or higher) declaration in the current scope.