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
	}
}