Use a scored grammar. For example, this scoring directive

 opcode: /$match_text1/  <score: { length join '' @item}>
 opcode: /$match_text2/  <score: { length join '' @item}>
 opcode: /$match_text3/  <score: { length join '' @item}>

would return the opcode with the longest length, as opposed to which
one matched first.

Just look for the section "Scored productions" in the .pod
documentation. 
