Validate by number? That's even easier. Seriously, go use Params::Validate when you want to do this sort of thing. Its simple enough for casual use and has enough bells and whistles to do heavy duty things too.

use Params::Validate 'validate_pos';

sub foo {
    # Three args are required.
    my ( $foo, $bar, $baz )
      = validate_pos( @_, 1, 1, 1 );

    ...
}
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