... why the third line is 3E0 and what does that stand for?
From the discussion of Range Operators in scalar context (the "flip-flop" operator):
The right operand is not evaluated while the operator is in the "false" state, and the left operand is not evaluated while the operator is in the "true" state. ... The value returned is either the empty string for false, or a sequence number (beginning with 1) for true. The sequence number is reset for each range encountered. The final sequence number in a range has the string "E0" appended to it, which doesn't affect its numeric value, but gives you something to search for if you want to exclude the endpoint.
[Emphases added]
So IOW, the final sequence number matches
qr{ \A \d+ E0 \z }xms
See also
The Scalar Range Operator ("Exluding Markers" section) and
Flipin good, or a total flop? (specifically
Re: Flipin good, or a total flop?), both in the Monastery's
Tutorials section.
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