Win8 Strawberry 5.8.9.5 (32)  Tue 03/16/2021 17:26:59
C:\@Work\Perl\monks\davebaker
>perl -Mstrict -Mwarnings

my $s = 'foo
bar';
print "A: >>$s<< \n";

my $search  = 'foo\nbar';   # note single quotes!
print "B: >>$search<< \n";  # \n is '\n'

my $replace = "hoo-ray";  # can be single/double quotes

$s =~ s/$search/$replace/;  # no /g - one replacement only
print "C: >>$s<< \n";

^Z
A: >>foo
bar<<
B: >>foo\nbar<<
C: >>hoo-ray<<

##</code><code>##

Win8 Strawberry 5.8.9.5 (32)  Tue 03/16/2021 17:28:26
C:\@Work\Perl\monks\davebaker
>type search.dat
foo\nbar

>perl -Mstrict -Mwarnings

my $s = 'foo
bar';
print "A: >>$s<< \n";

open my $fh, '<', 'search.dat' or die "opening: $!";

chomp(my $search = <$fh>);
print "B: >>$search<< \n";  # \n is essentially '\n'

my $replace = "hoo-ray";

$s =~ s/$search/$replace/;
print "C: >>$s<< \n";

^Z
A: >>foo
bar<<
B: >>foo\nbar<<
C: >>hoo-ray<<