One simple approach is to determine the highest key. Then loop over every possible key adding '-' if there is no value for that key. I have a feeling there may be a better way to do this for large datasets but on a small simple scale it works.
use strict;
use warnings;
my %data = (
1 => 'A',
2 => 'B',
3 => 'C',
6 => 'D',
9 => 'E',
10 => 'F',
);
my $max_key = 0;
for ( keys %data )
{
$max_key = $_ if $_ > $max_key;
}
for ( 1 .. $max_key )
{
$data{$_} = '-' unless $data{$_};
}
print "$_ : $data{$_}\n" for keys %data;
#Outputs
6 : D
3 : C
7 : -
9 : E
2 : B
8 : -
1 : A
4 : -
10 : F
5 : -