4.5. Private Variables in Subroutines But if Perl can give us a new @_ for every invocation,can't it give us variables for our own use as well? Of course it can. By default,all variables in Perl are global variables; that is,they are accessable from every part of the program. But you can create private variables called lexical variables at any time with the my operator: sub max {
my($m,$n); # new,private variables for this block
($m,$n) = @_; # give names to the parameters
if ($m > $n) { $m } else { $n }
}
These variables are private (or scoped) to the enclosing block; any other $m or $n is totally unaffected by these two. And that goes the other way,too; no other code can access or modify these private variables by accident or design. You could drop this subroutine into any Perl program in the world and know that you wouldn't mess up that program's $m and $n (if any). Inside the if's blocks,there's no semicolon needed after the return value expression. Though Perl allows you to omit the last semicolon in a block,in practice you omit it only when the code is so simple that you can write the block in a single line.
The subroutine in the previous example could be made simpler. Did you notice that the list ($m,$n) was written twice? The my operator can be applied to a list of variables enclosed in parentheses,so it's customary to combine those first two statements in the subroutine: my($m,$n) = @_; # Name the subroutine parameters
That one statement creates the private variables and sets their values,so the first parameter has the easier name $m and the second has $n. Nearly every subroutine will start with a line much like that one,naming its parameters. When you see that line,you'll know that the subroutine expects two scalar parameters,which you'll call $m and $n inside the subroutine. 
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