for (10,15,20831323,99,100) {
  print "$_: ", nearPrime($_), "\n";
}

sub nearPrime {
    my $num = shift;
    local $_ = 0; # otherwise tries to modify for loop var above
    return $num if isPrime($num);
    while (1) {
        $_++;
        return ($num - $_) if isPrime($num - $_);
        return ($num + $_) if isPrime($num + $_);
    }
}

...