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__RESULT__
0:10:1

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

__RESULT__
0:1:0:1

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

__RESULT__
0:1:0:1

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

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

__RESULT__
0:10:1