use Sort::Key::Multi 'iiikeysort'; 
  # the 'iii' stands for three integer keys

my (%month, $month);
$month{$_} = ++$month for qw(jan feb mar apr ...);

@sorted = iiikeysort { /^webadmin_(\w+)_(\d+)_(\d+)\.log$/;
                        ($3, $month{$1}, $2) } @filenames;