$s = "1. ...... AB_CD 1.1 ...... EF_GH 1.2 .... IJ_KL_MN 2. ............ OPQR";;

@a = map{ 
    /(\S+)\s+\.+\s+(\S+)/; 
    { chapter => $1, name => $2 } 
} split '(?<=[^.])\s(?=[^.])', $s;;

pp \@a;;
[
  { chapter => "1.", name => "AB_CD" },
  { chapter => "1.1", name => "EF_GH" },
  { chapter => "1.2", name => "IJ_KL_MN" },
  { chapter => "2.", name => "OPQR" },
]