Another variation on the theme:

#! perl
use strict;
use warnings;
use Data::Dumper;

my $str = "1. ...... AB_CD 1.1 ...... EF_GH 1.2 .... IJ_KL_MN 2. .....
+....... OPQR";
   $str =~ s/ \.{2,} //gx;
   $str =~ s/ \.\s   //gx;

my   %terms = split(/\s+/, $str);
my   @array;
push @array, { 'chapter' => $_, 'name' => $terms{$_} } for sort keys %
+terms;

print Dumper(\@array);
[download]

Output:

$VAR1 = [
          {
            'name' => 'AB_CD',
            'chapter' => '1'
          },
          {
            'name' => 'EF_GH',
            'chapter' => '1.1'
          },
          {
            'name' => 'IJ_KL_MN',
            'chapter' => '1.2'
          },
          {
            'name' => 'OPQR',
            'chapter' => '2'
          }
        ];
[download]

Hope that helps,