Hm. You have a big list and a small list of dates, both sorted (or readily sortable).

And for each date in the small list, you want to find the subset of the big list that are 1 year either side.

And you search anew for each item in the small list?

Here's a one-pass algorithm:

my @bigSorted = ...;
my @smallSorted = ...;
my %ranges;
my( $lo, $hi ) = ( 0, 0 );
for ( @smallSorted ) {
    my( $early, $late ) = calcBoundaryDates( $_ );
    ++$lo while $bigList[ $lo ] lt $early;
    $hi //= $lo;
    ++$hi while $bigList[ $hi ] le $late;
    $ranges{ $_ } = $hi - $lo;
}
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