Aʿshār relates to ʿushr, “a tenth” or a portion. Classical lexical usage also applies it to broken pieces of a vessel, enabling the picture of a heart reduced to fragments.
In fī aʿshāri qalbin, aʿshār is genitive after fī, and qalbin is its genitive complement. The arrows do not strike an intact heart but the pieces of one already broken.
The principal literary force is not a requirement to count precisely ten pieces. It is the sense of division and damage, giving new visual form to the suffering expressed earlier in qātilī.