__RESULT__
0:10:1

##</code><code>##

my @a = (':','',':','');
my $i=0;
print join $a[$i++], 0,1,0,1; # Wrong, $i isn't incremented

##</code><code>##

__RESULT__
0:1:0:1

##</code><code>##

print join $_ ? '' : ':', 0,1,0,1; # Wrong, $_ isn't set

__RESULT__
0:1:0:1  

##</code><code>##

(start) 
1. join $a[0] and $a[1]
2. join $a[1] and $a[2]
3. join $a[2] and $a[3] 
(finished)

##</code><code>##

my $j = '';
$j .= $_ ? length $j ? ":$_" : $_ : $_ for (0,1,0,1);
print $j;

__RESULT__
0:10:1