Using Unicode Character names (Creating Unicode) avoids your problem. It has the further advantage that it eliminates confusion between similar looking characters.

use strict;
use warnings;
use utf8;
use Test::More tests=>1;



my $string = "wjzw’xl cowuxze.";
my $required = "wjzw\N{APOSTROPHE}xl cowuxze.";     #"wjzw'xl cowuxze.
+";

$string =~ s/\N{RIGHT SINGLE QUOTATION MARK}/'/ ;


is( $string, $required, 'APOSTROPHE' );
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UPDATE: Added link