In the second case the sprintf statement is taken as the argument of the print statement, so the statement as a whole concatenates the result of print with the string "hello\n" and prints 00001 as a side effect.

print((sprintf "%5.5d", 1) . "hello\n");
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will do what you want.

Incidently, I'd use %05d as a formatting string: this is compatible with C, yours isn't if memory serves (and it doesn't really make sense if you think of it).

Hope this helps, -gjb-