my %month = (
    jan => 1,
    feb => 2,
    ...
);
m/webadmin_([^_]+)_(\d+)_(\d{4})\.log/;
my ($month, $day, $year) = ($1, $2, $3);
my $new_filename = sprintf "%04d-%02d-%02d", $year, $month{$month}, $day;