Attack Piece Block
From the Super Mario Wiki
Attack Piece Blocks are blocks from Mario & Luigi: Bowser's Inside Story with the letter "A" in them, most likely standing for "attack". They resemble pieces of a puzzle joined together to make a block. Attack Piece Blocks are scattered throughout Bowser's body and in places outside Bowser such as Dimble Wood. Blocks inside Bowser's body look two-dimensional, while blocks in the 3D outside world are 3D. There are ten Attack Piece Blocks in almost every area; they will give Mario and Luigi an Attack Piece if struck. Hitting all ten will give the bros. a new Special Attack. Sometimes Mario and Luigi can find several of these blocks all at once. Once in a while they can get all ten at once. Finding several at once could be found inside small 15-30 hit blocks. Bowser can not strike these blocks.
Attack Piece Blocks are set to return in the sequel Mario & Luigi: Dream Team.