An alternative to using substr() as an lvalue is to
             specify the replacement string as the 4th argument.
             This allows you to replace parts of the EXPR and
             return what was there before in one operation, just
             as you can with splice().
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The key phrase in the above being "one operation." When used as an lvalue it does it in two operations, the assignment and then the substr and you get the result of the substr().

Update: I was becoming more and more convinced that my guess was the correct one. Then I discovered this:

$ perl -le '$,="|";$s="foobar"; print substr($s,-2,2)="z",substr($s,-2
+,2)'
z|bz
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given that the substring is different in the two cases, it isn't just doing the substr() after the replacement. I'd say that is a bug and I think I have to agree with tye. It seems that the "right" behavior would be to return the string assigned as in any assignment.

I wonder how many obfus changing that will ruin...

-sauoq
"My two cents aren't worth a dime.";