~/naive-file.txt
~/old-text.txt
~/new-text.txt

$ cat ~/naive-file.txt
Sed id ligula quis est convallis tempor.

This is the old text.

It might have multiple lines and some special characters like these  { & % #)!
etc...


Nunc aliquet, augue nec adipiscing interdum, lacus tellus malesuada massa, quis
varius mi purus non odio.

$ cat ~/old-text.txt
This is the old text.

It might have multiple lines and some special characters like these  { & % #)!
etc...

$ cat ~/new-text.txt
This is the new text.

It could also have multiple lines and special characters like these  { & %
etc...

Sed id ligula quis est convallis tempor.

This is the new text.

It could also have multiple lines and special characters like these  { & %
etc...


Nunc aliquet, augue nec adipiscing interdum, lacus tellus malesuada massa, quis
varius mi purus non odio.

#!/usr/bin/perl
use warnings;
use strict;

open my $ot, '<', 'old-text.txt' or die $!;
chomp( my @lines = <$ot> );
open my $nt, '<', 'new-text.txt' or die $!;
my %replace;
@replace{@lines} = <$nt>;
chomp for values %replace;

my $regex = join '|', map quotemeta, @lines;
open my $in, 'naive-file.txt' or die $!;
while (<$in>) {
    s{(.*)}{$replace{$1} // $1}e;
    print;
}

my $regex = join '|', map quotemeta, sort { length $b <=> length $a } @lines;