Maybe this will explain it?

@a = qw[1 2 3 4 5];
print "@a";

1 2 3 4 5

splice @a, 1,3, qw[1.5 2 2.5 3 3.5 4 4.5];
print "@a";

1 1.5 2 2.5 3 3.5 4 4.5 5
[download]

The splice line above say's: replace elements @a[1,2,3] (offset 1, length 3) with the list that follows.

Using delete and assignment (or just assignment) you can replace 1 for 1 in the array. Whereas, using splice you can replace any number (length) of elements at any offset, with any number of replacement elements. More or less than the number being replaced.