There is nothing strange. The following altered code demonstrates that your $c variable contains 13 chars , which you want displayed within 11 space holders (1+10). Two chars werer choped, as per your request .

format STDOUT =
@<<<<@<<<<@<<<<<<<<<<END
$a, $b, $c
.

$a = "a";
$b = "b";
$c = "c         .bc";

print 'len=',length $c;
write(STDOUT);
[download]

Produces this output:

len=13
a    b    c         .END
[download]