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 : -