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

##</code><code>##

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

##</code><code>##

my @month = qw(jan feb mar apr may jun jul aug sep oct nov dec);
my %month = map { $_ => $month[$_] } (0..$#month);
...
$m_name = $month{$m_number};