CONFUSION reigned in the homes of the good and the great at the weekend when Piaget's managing director, Daniel Perel, decreed in the invitation to the Piaget Cup - one of the social engagements of the summer - that the dress code was ''evening attire''. Such was the puzzlement in one Mid-Levels household that the perplexed man of the house (and you know we couldn't make this up even if we tried) stood by his extensive wardrobe, mobile telephone at ear, and located one of Perel's minions at the Sha Tin racecourse so as to get clearer guidelines. Dapper Perel, himself, we thought adapted to his dictum in a novel sort of way - a pronounced five o'clock shadow set off with a colourful Versace scarf. And how would fashion arbiter Allan Zeman interpret ''evening attire'', we wondered? Wife Charmaine was her usual chic self, but the garment and property tycoon - of whom it has been said that it is easier for the Biblical camel to pass through the eye of a needle than it is to find a pair of socks in his closet - opted for a black suit. Black was also how hairdresser-by-invitation to the rich and famous Kim Robinson read the fashion code. ''Black is now the thing,'' he stated as he struggled to cope with the ladies who were lining up to airbrush his cheeks - and thereby ensure that any secrets they might have spilled under the heat of his blow-dryer were safe. One taipan disclosed that since he usually wore pyjamas in the evening he was somewhat puzzled by the dress requirements. ''But don't you dare quote me on that,'' he implored. Since we are always respectful of people's wishes on Keeping Posted we assured David Davies that we would adhere to his request. Hearing the word ''pyjamas'' one socialite pricked up her ears and sauntered towards Keeping Posted. ''Pyjamas,'' she cooed, ''is what my darling husband keeps under his pillow in case of a fire.''