#!/usr/bin/perl

# https://perlmonks.org/?node_id=1227916

use strict;
use warnings;

##################### configuration section

my $section = 'ObjectType1';
my %changes = ( Param1 => 0, Param2 => 'SomeOtherText', Param3 => 'Foobar');

##################### end configuration section

my $allkeys = join '|', keys %changes;
my $pattern = qr/\b($allkeys)\b/;
local $/ = "\n}\n";
while( <DATA> )
  {
  if( /\b$section\b/ )
    {
    my @context;
    print $& while
      @context && $context[-1] eq $section && /\G(\h*$pattern = ).*\n/gc ?
        "$1$changes{$2}\n" =~ /.*/s :
      @context && /\G\h*\}\n/gc ? pop @context :
      /\G\h*([\w ]+)\n\h*\{\n/gc ? push @context, $1 :
      /\G.*\n/gc;
    }
  else
    {
    print;
    }
  }

__DATA__
ObjectType1
{
   Param1 = 8
   NestedObject
   {
      Param1 = 3
      Param2 = SomeText
   }
   Param2 = SomeText
}
ObjectType2
{
   Foo
   {
      Param1 = StaySame
      ObjectType1
      {
         Param3 = ReplaceThis
      }
   }
}
ObjectType1
{
   ...
}

##</code><code>##

ObjectType1
{
   Param1 = 0
   NestedObject
   {
      Param1 = 3
      Param2 = SomeText
   }
   Param2 = SomeOtherText
}
ObjectType2
{
   Foo
   {
      Param1 = StaySame
      ObjectType1
      {
         Param3 = Foobar
      }
   }
}
ObjectType1
{
   ...
}