Just add half a second to the time, then truncate to the second.

use DateTime::Format::Strptime qw();

DateTime::Format::Strptime
    ->new(pattern => '%T.%N')
    ->parse_datetime('15:59:57.203')
    ->add(nanoseconds => 0.5 * 1000 * 1000 * 1000)
    ->truncate(to => 'second')
    ->strftime('%T');
# returns '15:59:57'

DateTime::Format::Strptime
    ->new(pattern => '%T.%N')
    ->parse_datetime('15:59:57.703')
    ->add(nanoseconds => 0.5 * 1000 * 1000 * 1000)
    ->truncate(to => 'second')
    ->strftime('%T');
# returns 15:59:58
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