$_       = 'foo bah bar';
$find    = 'bah';
$replace = '[\1]';
s/(\Q$find\E)/$replace/g;

$\ = "\n";
print;      # "foo [\1] bar" and not "foo [bah] bar"

##</code><code>##

$_       = 'foo bah bar';
$find    = 'bah';
$replace = '[\1]';

$idx = index($_, $find);
substr($_, $idx, length($find), $replace) if $idx >= 0;

$\ = "\n";
print;

##</code><code>##

$_       = 'foo bah bah bar';
$find    = 'bah';
$replace = '[\1]';

$idx = length($_);
while (($idx = rindex($_, $find, $idx-1)) >= 0) {
   substr($_, $idx, length($find), $replace);
}

$\ = "\n";
print;