That's pretty clever, I like it! Go one step further and define named constants, and it's even readable:

use constant USERNAME => 0;
use constant LOGIN    => 1;
use constant LOGOUT   => 2;
use constant ACTIVITY => 3;

format admin_usage_data =
@<<<<<<<    @<<<<<<< @<<<<<<<  @<<<<<<<<<<<<<<<<<<<<<<<<<<
$_[USERNAME], $_[LOGIN], $_[LOGOUT], $_[ACTIVITY]
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