You've been given good answers already, but let me just add some details on the way the regex engine processes this string.

According to me, it should be "$1= This is Perl".

In fact, that's what is initially happening: the regex engine sees (.+) and grabs the whole string, i.e. "this is Perl". But then, it sees (e|r), so, in order for the whole regex to be successful, it has to backtrack and give back "l" and then "r", so that (e|r) can be successful. Note that this would happen even if you did not have capturing parentheses, so that the point is not so much that it is trying to populate $2, but that (e|r) has to match something for the whole regex to be successful.

Once it has matched the "r" with the second capture, the last part of the regex, (.*)$ can match the "l".