The first form your your "solution" does not work - it doesn't print the newline. This is for the very same problem you're suffering in your first try: the parentheses are interpreted by a function call to print.

If you put warnings on:

perl -wce 'print ( (split "\t", $line)[0] ), "\n" ;';

__output__
print (...) interpreted as function at -e line 1.
Useless use of a constant in void context at -e line 1.
-e syntax OK
[download]

where the "constant in void context" is the newline char. The problem is obviously corrected in your second solution (this time without quotes :), because all elements become arguments to the print function.