Why do you test $i > 0 - it will always fail on the first iteration and you never get another. More likely the logic of what you want is:

use strict;
use warnings;

for my $avg (0, 1.3, 1.5, 1.6, 3, 5) {
    my $average = $avg;
    my $stars = 0;
    
    print "$average: ";
    
    while ($stars < int ($average)) {
        print "*";
        ++$stars;
    }
    
    if ($stars <= $average - 0.5) {
        print "+";
        ++$stars;
    }
    
    while ($stars < 5) {
        print ".";
        $stars++;
    }
} continue { 
    print "\n";
}
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