There's an example at the bottom of Algorithm::Knapsack. For weights, you'd use the size of the files and for the capacity, you'd use the capacity of a DVD.

Solution:

[ almust says this doesn't work. See Re^3: I want to maximize data storage in a DVD ]

use strict;
use warnings;

use Algorithm::Knapsack qw( );

use constant DVD_CAPACITY => ...;

# Replace this function
sub process_dvd {
   my ($dvd) = @_;
   use Data::Dumper qw( Dumper );
   local $Data::Dumper::Useqq  = 1;
   local $Data::Dumper::Terse  = 1;
   local $Data::Dumper::Indent = 1;
   print(Dumper($dvd));
}

sub fill_bag {
   my ($bag_size, $items) = @_;

   my @weights;

   for my $size (keys(%$items)) {
      push @weights, ($size) x @{$items->{$size}};
   }

   return undef if !@weights;

   my $knapsack = Algorithm::Knapsack->new(
      capacity => $bag_size,
      weights  => \@weights,
   );

   $knapsack->compute();

   my $solution = ( $knapsack->solutions() )[0]
      or die("No solution\n");

   my @bag;
   for my $size (@$solution) {
      my $item = shift( @{$items->{$size}} );
      push @bag, $item;
   }

   return \@bag;
}

{
   my %files;

   for my $file (...) {
      my $size = -s $file;
      push @{ $files{$size} }, $file;
   }

   while (my $dvd = fill_bag(DVD_CAPACITY, \%files)) {
      process_dvd($dvd);
   }
}
[download]

Untested.

Update: Added solution.
Update: Fixed solution.