Because it is easier for maintenance to name them from the beggining.

Suppose you use my (undef, $d, undef, $f) = @_; and 2 months later you also need another variable. Then you need to go look to what code calls is then try to add the argument there - or find another way of passing it - and ultimetly find out it is already being passed as the 3th argument.

A real live example is from a mod_perl framework that uses: my ($data, $p, $r, $dbh) = @_;.
The explenation of the variables:

• $data: The object of a data class that is linked with the module,
• $p: A list of common arguments (who is logged in, what language, what level, ...),
• $r: The Apache::Request object,
• $dbh: The database connection,

While most of the code will never use $r and $dbh it is still useful to store them in a lexical since it is immposible to predict how the subroutine will grow and when it will need them.

What you call an advantage I call a serious disadvantage for maintenance. If you later need $r for some reason you need to look up as which argument it is passed.

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