Here's another thread which might help explain things: Regular expression "replace string interpolation" problem.

What confuses a lot of people is that when the replacement is a simple expression like $that, the following are equivalent:

s/$this/$that/;
s/$this/$that/e;
[download]

However, note how this changes when the replacement varies just a little:

# these are are equivalent:
s/$this/some text/;
s/$this/"some text"/e;

# and so are these:
s/$this/$that was close/;
s/$this/$that." was close"/e;
[download]