# Handles initial parse (gets field
# names from first line of 'file.out'
my $file = ParseFile('file.out');    

# Specifies fields on which to subtotal
$file->keys = qw( Cust_Grp Order_Date Item Name );

# Adds a field
$file->field('Line_Total', sub { $row->{Unit_Price}*$row->{Qty} });

...

# Process file
$file->process();