How about 'mod'? 
$direction = 1; #set to -1 to go to opposite direction

@array = split(//, $string);
for (@array) {
   print $array[$i];
   $i = ($i + $direction) % ($#array + 1);
};

[download]
You have to initialize $i.

UPDATED: Added $direction (I think I was going backwards first)

In reply to Re: PUZZLED by Punto
in thread PUZZLED by Anonymous Monk

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