FAQ: What's the difference between deep and shallow binding?

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7.18: What's the difference between deep and shallow binding?

    In deep binding, lexical variables mentioned in anonymous subroutines
    are the same ones that were in scope when the subroutine was created. In
    shallow binding, they are whichever variables with the same names happen
    to be in scope when the subroutine is called. Perl always uses deep
    binding of lexical variables (i.e., those created with my()). However,
    dynamic variables (aka global, local, or package variables) are
    effectively shallowly bound. Consider this just one more reason not to
    use them. See the answer to the section on "What's a closure?".


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