Take a look at perlsub--specifically, the sections "Private Variables via my()" and "Temporary Values via local()".

my() variables aren't stored in the symbol table; local() variables are stored in the symbol table, in the sense that they give "local values to global (package) variables."

From perlsub:

    In general, you should be using ``my'' instead
    of ``local'', because it's faster and safer.
    Exceptions to this include the global punctuation
    variables, filehandles and formats, and direct
    manipulation of the Perl symbol table itself.
    Format variables often use ``local'' though, as
    do other variables whose current value must be
    visible to called subroutines.
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