MooseX/Types/ISO8601.pm exerpt

{
    my %coerce = (
        ISO8601TimeDurationStr, 'PT%02HH%02MM%02S.%06NS',
        ISO8601DateDurationStr, 'P%02YY%02mM%02dD',
        ISO8601DateTimeDurationStr, 'P%02YY%02mM%02dDT%02HH%02MM%02S.%06NS',
    );

    foreach my $type_name (keys %coerce) {

        my $code = sub {
            my $str = DateTime::Format::Duration->new(
                normalize => 1,
                pattern   => $coerce{$type_name},
            )
            ->format_duration( shift );

            # Remove fractional seconds if there aren't any.
            $str =~ s/\.0+S$/S/;
            return $str;
        };

        coerce $type_name,
        from Duration,
            via { $code->($_) },
        from Num,
            via { $code->(to_Duration($_)) };
            # FIXME - should be able to say => via_type 'DateTime::Duration';
            # nothingmuch promised to make that syntax happen if I got
            # Stevan to approve and/or wrote a test case.
    }
}