#!/usr/bin/perl

# https://perlmonks.org/?node_id=1220734

use strict;
use warnings;

my $new = <<END;
Connection Culv=This is Line3 - New text here
     333 333 This should be new too
Conn Culvert Barrel=Culvert3
*
Connection Culv=This is Line1 - New text here
     111 111 This should be new too
Conn Culvert Barrel=Culvert1
*
Connection Culv=This is Line2 - New text here
     222 222 This should be new too
Conn Culvert Barrel=Culvert2
*
END

my $model = <<END;
Connection Culv=This is Line1
     111 111 
Conn Culvert Barrel=Culvert1 
* 
Connection Culv=This is Line2 
     222 222 
Conn Culvert Barrel=Culvert2 
* 
Connection Culv=This is Line3 
     333 333 
Conn Culvert Barrel=Culvert3 
*
END

my %replace;
$replace{$2} = $1 while
  $new =~ /^Connection Culv=(.*\n.*\n)(Conn Culvert Barrel=.*\S)/gm;
$model =~ s/^Connection Culv=\K.*\n.*\n(?=(Conn Culvert Barrel=.*\S))/
+$replace{$1}/gm;

print $model;
[download]

Outputs:

Connection Culv=This is Line1 - New text here
     111 111 This should be new too
Conn Culvert Barrel=Culvert1 
* 
Connection Culv=This is Line2 - New text here
     222 222 This should be new too
Conn Culvert Barrel=Culvert2 
* 
Connection Culv=This is Line3 - New text here
     333 333 This should be new too
Conn Culvert Barrel=Culvert3
[download]