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;
}