I'd use rindex to find the last * before position 30. Note that you don't split on /\*/ but on /\* /.

You can use the look-behind (?<= to specify that the space is preceded by that many characters:

#!/usr/bin/perl
use strict;
use warnings;

my $string = "hello my * name is Rob * and * I am a very nice person *
+ at least * I think * so";

my $part1 = "hello my * name is Rob * and *";
my $part2 = "I am a very nice person * at least * I think * so";

sub split_before {
    my ($string, $pos) = @_;
    my $pos = rindex $string, '*', 30;
    split /(?<=^.{$pos}\*) /, $string, 2
}

use Test2::V0;

is "$part1 $part2", $string;

is [split_before($string, 30)],
    [$part1, $part2];

done_testing();
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BTW,

    (split /(^.{,30}\* )/, $string, 2)[1, 2]
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passes the first test, too (but not the second one, as the space is still part of $part1).

Update: Read below for a fix, thanks AnomalousMonk. I shouldn't work on two different tasks at the same time.