perldoc -f split

Not possible to be positive without your data, but suspect it is like this:

This is my first line. This is my second line. This is....

Oh!
See the space after each period.
Guess what your split does with it.
Right!

and, in apology for the flippancy of the above, a confession: I puzzled considerably over the same thing when I first encountered it. FWIW, a good solid understanding of regexen will make using split (and perl, in general) easier, more productive, and a darn-sight more fun.