Yeah, his docs are always novels :) see https://metacpan.org/pod/Perl6::Form#Declarative-field-widths, its what hes talking about ... this example might be clearer

field of length ten  {<<<<<<<<<}

field of length ten with CHECKSUM  {<<(10)<<<} basically the number has to match the literal length of the field including every single character like {}, basically length of the string , if it isn't, it errors (programmer made typo fix it programmer fix it)

field of length ten with TELLSUM  {<{10}<}, the length of this string is 8 characters, but the { curly braces } mean don't count characters for length, just use the number, so the field width is 10

field of length ten with TELLSUM  {<<<<<<<{10}<<<<<<<}, the length of this string is 20 characters, but the { curly braces } mean don't count characters for length, just use the number, so the field width is 10