This doesn't work: 
 my $result= do { $result += $_  foreach (@items) };

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because $result isn't defined inside the do block. Yet, something like read (FH, my $buffer, 999) works just fine, even though this would seem to be the same situation: the variable is defined inside the expression where it is used.

It sounds more like a lexer issue, in that the name isn't known elsewhere in the expression, even though the variable is in fact defined! I've not explored that hypothosis, but wonder if anyone already knew.

—John

In reply to when does the definition of a my variable take effect? by John M. Dlugosz

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