it is not so clear to me: perhaps you should post an example of the intended final datructure, but..

if you are trying to understand the .. part, in this case it is named flip-flop operator described among Range-Operators in the perlop

To help you understand it run the following code

use strict;
use warnings;

my $part = 0;

while (<DATA>){
    chomp $_;
    if ( $_ =~ /objet => debut$/ .. /objet => fin$/){
        print "TRUE\n";
        if ( /objet => debut$/ ){
            next;    # we skip the opening tag
        }
        elsif ( /objet => fin$/ ){
            $part++; # we increment our index $part on closing tag
        }
        else{
            print "\tPART $part: [$_]\n"; # here is the meat: do what 
+you need
        }
    }
    else{
        print "FALSE for line: [$_]\n"; 
    }
}

__DATA__
objet => debut
index => 1
a => "premiere valeur"
z => "dernier mot"
objet => fin
ALIEN LINE AFTER fin
objet => debut
index => 77
a => "autre valeur"
z => "aurai-je le dernier mot ?"
objet => fin
[download]

..and you will see how it works:

TRUE
TRUE
        PART 0: [index => 1]
TRUE
        PART 0: [a => "premiere valeur"]
TRUE
        PART 0: [z => "dernier mot"]
TRUE
FALSE for line: [ALIEN LINE AFTER fin]
TRUE
TRUE
        PART 1: [index => 77]
TRUE
        PART 1: [a => "autre valeur"]
TRUE
        PART 1: [z => "aurai-je le dernier mot ?"]
TRUE
[download]

L*