This may be an ugly hack, but it works:

$pad = ' ' x 80;  ## make the length the same as your max expected

$c = "test test test";
$fmt = "A4 x1 A4 x1 A4 x1 A4";
@c = unpack($fmt, "$c$pad");
print join ("*",@c), "\n";

$d = "test test test ";
$fmt = "A4 x1 A4 x1 A4 x1 A4";
@d = unpack($fmt, "$d$pad");
print join ("*",@d), "\n";
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--Jim

Update: BTW, I would avoid using $a as a variable since it (and $b) has an evil association with sort. Check the link for details.