use Modern::Perl;
use Data::Dumper;

my @array;
my $str =
  "1. ...... AB_CD 1.1 ...... EF_GH 1.2 .... IJ_KL_MN 2. ............ OPQR";

while ( $str =~ /(\d+\.\d*)\s+\.+\s+(\S+)\s*/g ) {
	push @array, { chapter => $1, name => $2 };
}

say Dumper \@array;