#!/usr/bin/env perl

use warnings;
use strict;
use Data::Dump qw/dump/;

$_ = do { local $/; <DATA> };
        
dump { map { /(.+)\n/ => [ /^interesting line (.+?)$/mg ] }
       grep length, split /^Index: /m }
        
__DATA__
Index: file1.txt
Bunch
of
junk
interesting line 1
more junk
interesting line 2
more junk
Index: subdir/file2.txt
interesting line A1
still more junk
interesting line A2
last of the junk