#! perl -slw
use strict;
use Data::Dumper;

my $string = "foo[1].bar.baz[0] = 123";
my @bits = split '\.| = ', $string;

my $data = pop @bits;
m/(\w+)(?:\[(\d+)\])?/ 
and $data = { $1 => $2 ? [ (undef) x $2, $data ] : $data }
    for reverse @bits;

print Dumper $data;
__END__
P:\test>393104
$VAR1 = {
          'foo' => [
                     undef,
                     {
                       'bar' => {
                                  'baz' => '123'
                                }
                     }
                   ]
        };
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