You appear to be using an obsolete interface. 'American' no longer exists (and it should be 'British' anyway), and num2en is much more appropriate than the object.

Here's how it would look like using a modern vesion of the module, including the addition of pounds and pence as per the OP's request.

use Lingua::EN::Numbers qw( num2en );

my $en = num2en(313721.23);

my ($pounds, $pence) = split(/ point /, $en);

if ($pounds eq 'One') {
   $pounds .= ' pound';
} else
   $pounds .= ' pounds';
}

$pence ||= 'Zero';
$pence .=  ' pence';

$en = "$pounds and $pence";

print("$en\n")
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Untested.