First tested:
-------------
$str = 'abac 123 afa123f';
$str =~ s|\B123\B|***|g;
print $str;
output i got as i expected:
---------------------------
abac 123 afa***f


Second tested:
--------------
$str = 'abac ~~~ afa~~~f';
$str =~ s|\B\&\#x0007E\;\&\#x0007E\;\&\#x0007E\;\B|~~~|g;
print $str;
output i got:
-------------
abac ~~~ afa~~~f
expected output:
----------------
abac ~~~ afa~~~f