$cowtime->timestamp is a DateTime object, so you could use the following:

$cowtime->timestamp->strftime("%Y-%m-%dT%H:%M:%S.%9N%z") =~ s/(?=..\z)
+/:/sr
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I'm not aware of any DateTime::Format::* class that would produce the format you want. If there was you could set it as the default format (used for stringification) using the following:

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

my $format = DateTime::Format::Foo->new(...);
$cowtime->timestamp->set_formatter($format);
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