use strict;
use warnings;

my $f1 = '1.txt';
my $f2 = '2.txt';

my (@strings, $text);

open (FH, $f1) || die;
chomp(@strings = <FH>);

open (FH, $f2) || die;
$text = join '', <FH>;
$text =~ s/\Q$_\E/******/g for @strings;
### Make sure it's treated as string and not expression

open (FH, ">$f2") || die;
print FH $text;
close FH;
[download]

1.txt:

find
search
[download]

2.txt:

a bunch
of words
with
find and
search and
some other words
[download]

2.txt after run:

a bunch
of words
with
****** and
****** and
some other words
[download]