A problem I see with this is there are two types of builder this could be used for:

• Simple ones. For example, inverting the following builder:

  sub _build_net_income {
    my $self = shift;
    return $self->gross_income - $self->taxes;
  }
  [download]

  to automatically create _build_gross_income and _build_taxes.

• Complex ones. For example, inverting the following:

  sub _build_net_income {
    my $self  = shift;
    my $gross = $self->gross_income;
    my $income_tax = do {
         if ($gross <   6_475) { 0 }
      elsif ($gross <  43_875) { (0.2 * ($gross - 6_475)) }
      elsif ($gross < 150_000) { (0.2 * (43_875 - 6_475))
                               + (0.4 * ($gross - 43_875)) }
      else                     { (0.2 * (43_875 - 6_475))
                               + (0.4 * (150_000 - 43_875))
                               + (0.5 * ($gross - 150_000)) }
      };
    my $child_tax_credit = 545 * scalar @{$self->children};
    my $taxes = $income_tax - $child_tax_credit;
    $taxes = 0 if $taxes < 0;
    return $gross - $taxes;
  }
  [download]

  to automatically create _build_gross_income and _build_children.

In the case of complex builders, I don't think your module has much chance of success. In the case of simple ones, you're not saving me much work.