COW is normally used for the copy for $& and friends, but it wasn't used in this case because there's not enough space in the buffer

Thanks. I think it follows that appending a character practically poisons a string in Perl (try 1e7 if on modern PC):

$ time perl -e '$_ = "a" x 1e6; $_ .= "a"; 1 while /./g'

real    0m35.868s
user    0m35.857s
sys    0m0.008s
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