from perldoc -f my:

my EXPR : ATTRIBUTES A `my' declares the listed variables to be local (lexically) to the enclosing block, file, or `eval'. If more than one value is listed, the list must be placed in parentheses. See the Private Variables via my() entry in the perlsub manpage for details.

You can find the aforementioned reference here: perlman:perlsub.

Update: My apologies, I misread the question and thought lorenw14 was asking about the scope of lexical variables.

--sacked