That could be modified easily enough to work with the real sentence number.

Computed data:

my %pad = ( 1 => 0, 4 => 3, 9 => 4, 13 => 6 );
my $longest = 6;
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Input:

my $sentence = 11;
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Outputs:

my $padded = sum map $pad{$_}, grep $sentence>=$_, keys %pad;
my $chapter    = int( $padded / $longuest ) + 1;
my $in_chapter =      $padded % $longuest   + 1
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Downside: it went from O(1) to O(Chapter). That shouldn't matter unless you have an incredible amount of chapters. In that case, convert the hash to an array and use a binary search to drop it to O(log Chapter).

Update: Expanded on the downside.