How should Regexes be passed to MAIN in Raku?

Is there an idiomatic way to pass a Regex from the command line to a Raku program. The following code accomplishes what I'm trying to do but it has a few issues (see below).

#!/bin/raku

use MONKEY-TYPING;
augment class Str {
  method Regex { 
    my $tmp = self.Str;
    return rx/ <$tmp> /;
  }
}

sub MAIN (Regex(Str) $sample) {
  if ( "hello world" ~~ $sample ) {
    say "hello world";
  }
}

Issues

1. This changes the default Str class which may cause unexpected behavior down the road.
2. Regexes passed to this work but sometimes act strange. For example:

this works:

$ ./cliRegex.raku '.*'
hello world

this does not work:

$ ./cliRegex.raku .*
Usage:
  ./cliRegex.raku <sample>

The below code checks if a string is a valid regex. It doesn't prevent the code injection mentioned by librasteve. It is simply intended to demonstrate using subset for validating a regex passed to MAIN in a file named cliRegex.raku.

#! /bin/raku

# returns true if needle is a valid regex
sub regexIsValid ( Str $needle ) returns Bool {
  try {
    "abcd" ~~ rx/ <$needle> /;
    return True;
  }
  die "Invalid regex: '$needle'\n$!\n\nSee `cliRegex.raku --help` for usage";
}

subset Validregex of Str where regexIsValid($_);

sub MAIN (
  Validregex $sample #= pass regex to sample to test if it's a regex 
) {
  if ( "hello world" ~~ rx/ <$sample> / ) {
    say "hello world";
  }
}

Examples

Valid Regex Passed

$ raku ./cliRegex.raku '.*'
hello world

Invalid Regex Passed

$ raku ./cliRegex.raku '*'
Invalid regex: '*'
Quantifier quantifies nothing

See `program.raku --help` for usage
  in sub regexIsValid at ./cliRegex.raku line 9
  in block <unit> at ./cliRegex.raku line 12