App::SimpleScan::Substitution::Line is a line with optional fixed variable values.
my $line =
App::SimpleScan::Substitution::Line->new(" this ");
# Use only this value when substituting "".
$line->fix('substituite' => 'change');
# what vars are fixed?
my @fixed_ones = $line->fixed();
# Forget about now.
# Forget all fixed variables.
# Get the line back as a string.
App::SimpleScan::Substitution::Line allows us to associate fixed substitution values with a specific string. This allows us to re-substituted the same value if, during string substitution, we find the variable reappearing as the result of substituting some other variable. This eliminates the "cross-product" bug that appeared in some complex nest substitions.