my $str = 'Hello World!';

# Alias $_ to $str.
foreach ($str) {
   # Assumes each substitute operator is on a different line.
   while (defined(my $code = <DATA>)) {
      eval($code)
         or die("Bad code at input line $.: $@\n");
   }
}

print("$str\n");  # [hello world!]

__DATA__
s/^/[/g
s/$/]/g
s/([A-Z])/lc($1)/eg