use warnings;
use strict;

# $sep needs to be some char guaranteed not to be in the section name
# (not the most elegant solution, but this isn't meant to be)
my $sep = "\n";
my $target_sec = "ObjectType1${sep}NestedObject";
my $target_param = "Param1";
my $replace_val = "Hello";

my ($secname,@sec_stack,$cur_sec);
while (<DATA>) {
	next if /^\s*#/ || !/\S/;
	if ( /^ (\s*) (.+?) \s* = \s* (.+?) \s* $/x ) {
		die "section name without a following block"
			if defined $secname;
		my ($indent,$param,$val) = ($1,$2,$3);
		if ($cur_sec eq $target_sec && $param eq $target_param)
			{ $_ = $indent.$param.' = '.$replace_val."\n" }
	}
	elsif ( /^ \s* (\w+) \s* $/x ) {
		die "two section names following one another"
			if defined $secname;
		$secname = $1;
	}
	elsif ( /^ \s* \{ \s* $/x ) {
		die "'{' without name" unless defined $secname;
		push @sec_stack, $secname;
		$cur_sec = join $sep, @sec_stack;
		$secname = undef;
	}
	elsif ( /^ \s* \} \s* $/x ) {
		die "section name without a following block"
			if defined $secname;
		die "'}' without '{'" unless @sec_stack;
		pop @sec_stack;
		$cur_sec = join $sep, @sec_stack;
	}
	else { die "Failed to parse '$_'" }
	print $_;
}

__DATA__
# comment
ObjectType1
{
	Param1 = Foo
	NestedObject
	{
		Param1 = Bar
	}
	# just another comment
}

ObjectType2
{
	Param1 = Quz
	NestedObject
	{
		Param1 = Baz
	}
}

##</code><code>##

use warnings;
use strict;
use Data::Compare qw/Compare/;

my @target_block = ('Object Type1','NestedObject');
my $target_param = 'Param 1';
my $new_val = 'Hello!';
my $data = do { local $/; <DATA> };

my @stack;
pos($data)=0;
while ( pos($data)<length($data) ) {
	use re '/msx';
	my $repl;
	if ( $data=~m{\G ^ \h* \# [^\n]* (?:\z|\n) }gc )
		{} # comment, nothing to do
	elsif ( $data=~m{\G \s* ( \w(?:[\w\h]*\w)? ) \s* \{ \h*\n* }gc )
		{ push @stack, $1 }
	elsif ( $data=~m{\G (?<pre> ^\h* ) (?<name> [^\n=]+?)
			(?<mid> \h*=\h* ) (?<value> [^\n]+? )
			(?<post> \h*(?:\z|\n) ) }gc ) {
		if ( Compare(\@stack,\@target_block)
				&& $+{name} eq $target_param ) {
			$repl = $+{pre}.$+{name}.$+{mid}.$new_val.$+{post};
		}
	}
	elsif ( $data=~m{\G \s* \} \h*\n* }gc ) {
		die "'}' with no opening '{'?" unless @stack;
		pop @stack;
	}
	else { die "Failed to parse at: \""
		.substr($data, pos $data, 50)."...\"" }
	print $repl//substr($data, $-[0], $+[0]-$-[0]);
}

__DATA__
# comment
Object Type1 {
	Param1 = Foo
	NestedObject {
		Param 1 = Bar   
	}
	# just another comment
}

# comment
ObjectType2 {
	Param1 = Quz = z
	NestedObject {
		Param1 = Baz
} }

##</code><code>##

use warnings;
use strict;
use Regexp::Grammars;

my @blockstack;
my $grammar = do { use Regexp::Grammars; qr{
	\A (?: <.comment> | <[confblock]> )* \z
	<rule: confblock>
		^ <blockname=([^\s\{\}=]+)> \{
		(?: <[param]> | <[confblock]> | <.comment> )*
		\} <.ws>
	<rule: param>
		^ <name=([^\n=]+?)> = <value=([^\n]+?)> (?:\n|\z)
	<token: comment>
		^ \h* \# [^\n]* (?:\n|\z)
}xms };

my $data = do { local $/; <DATA> };
$data =~ $grammar or die "failed to parse";
my %conf = %/; #/

use Data::Dump;
dd \%conf;

__DATA__
# comment
ObjectType1 {
	Param1 = Foo
	NestedObject {
		Param1 = Bar   
	}
	# just another comment
}

# comment
ObjectType2 {
	Param1 = Quz
	NestedObject {
		Param1 = Baz
} }