As you are reading from the file handle in list context, you advance its position until eof.

Then you print. At the former eof.

(If you had opened with +>>, you would not have been able to read anything without seeking first.)

Update: For what you want, and with a tiny bit more clarity:

open (A, '+<', "somefile.html") or die $!;
s/a/b/g for my @records = <A>;
seek(A, 0, 0);
print A @records;
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