open FILE,"> hello.txt" or die $!;

... I don't know how the open shown in your OP could work, period, given the missing close-quote, comma operator, space, open-quote sequence.

That's the old, disparaged, denigrated, frowned-upon, but not deprecated two-argument form of open. The file-access specifier is fused with the file name in a single string. IIRC, the space between the  '>' write-access specifier and the file name is just the sort of gateway (among others) to a potential pitfall that the three-argument open was designed to avoid. Yes, it works.