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#!/usr/bin/perl -w

...
    print "Maths:\n";
    print "\t$number % 9 || 9 = ", $number % 9 || 9, "\n\n";
}

=== Test: 1 - Number: 5943 ===
Brute force:
...
        1 + 0 = 1
Maths:
        2170 % 9 || 9 = 1