It will *happen* to work for some invalid inputs. Here are cases that support what I said:

Non-letters in $repl:

my $find= "find";
my $repl= "@@@@";

$_ = "Find";
print;
s/\b($find)\b/ uc($repl) | ( $1 ^ uc($1) ) /eig;
print;  # @```
        # Should be @@@@
[download]

Non-letters in $find:

my $find= chr(1234) . 'find';
my $repl= "aaaaa";

$_ = chr(1234) . 'FiNd';
print;
s/\b($find)\b/ uc($repl) | ( $1 ^ uc($1) ) /eig;
print;  # AAaAa
        # Should be aAaAa
[download]