What is the easiest or most effective way to convert month's abbreviation to a number in Perl? (ie “jan” to 1)

%mon2num = qw(
    jan 1  feb 2  mar 3  apr 4  may 5  jun 6
    jul 7  aug 8  sep 9  oct 10 nov 11 dec 12
);

$mon2num{"jan"}

my %number_for = (
    jan => 1,
    feb => 2,
#etc...
);
#...
do_something_with($number_for{$month})

my %month; @month{qw/jan feb mar apr may jun
                     jul aug sep oct nov dec/} = (1 .. 12);

my %month_num = do { my $i = 1; map {; $_ => $i++ } (
    qw( jan feb mar apr may jun jul aug sep oct nov dec )
) };

my %month_num;
$month_num{ $_ } = 1 + keys %month_num for (
    qw( jan feb mar apr may jun jul aug sep oct nov dec )
);

my %month_num = do {
    my @month = qw( jan feb mar apr may jun jul aug sep oct nov dec );
    zip2( 1 .. 1+$#month, @month );
};

# create a lookup table of month abbreviations to month numbers
my %month_abbr_to_number_lkup = (
    jan => 1,
    feb => 2,
    mar => 3,
    apr => 4,
    may => 5,
    jun => 6,
    jul => 7,
    aug => 8,
    sep => 9,
    oct => 10,
    nov => 11,
    dec => 12,
);

# get the number for a month
my $number = $month_abbr_to_number_lkup{$abbr}
    || die "Could not convert month abbreviation '$abbr' to a number.";

%mon_2_num = (jan => 1,
              feb => 2,
              ...);

$month_number = $mon_2_num{lc($month_name_abbrev)};

@month{qw(jan feb mar apr may jun jul aug sep oct nov dec)} = 1..12;

my @months = qw(Jan Feb Mar Apr May Jun
                Jul Aug Sep Oct Nov Dec);
my %monthnum = map { $_ => $months[ $_ - 1 ] } 1..12;