I think you have an excellent start. You can simplify it a fair bit, however:

    my $pre = (shift) % 100;
    my $amble = (shift) % 100;
    my $post = (shift) % 100;
    $pre = 100 - $amble   if( $amble + $pre > 100 );
    $post = 100 - ($amble + $pre);
    print "$pre + $amble + $post = ",
        $pre + $amble + $post , "\n";
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That should produce the same results modulo round-off errors.

        - tye (but my friends call me "Tye")