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

# versus ...

my %is_month_abbrev;
@is_month_abbrev{ qw( jan feb mar apr may jun jul aug sep oct nov dec ) } = (undef) x 12;

# or, with an intermediate array ...

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