Overhead how? Depending on what I'm doing, hash-slicing might be harder to maintain than map.

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;
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Often, losing a few micro-seconds is worth readability and maintainability. In both the slicing solutions, I either needed a separate array or hard-coding a magic number. (The number of months in a year isn't always a given. Some calendars have as few as 10, others have up to 14, and still others vary from year to year.)