It's quicker to code it than to explain it:

use strict;

my @dirs = qw(dir1 dir2 dir3);

my @files;
for my $dir ( @dirs ) {
  opendir my $dh, $dir or die "opendir failed: $dir ($!)\n";
  push @files, grep -f $_, readdir $dh;
}

my @sorted =
  map $_->[ 0 ],
  sort { $b->[ 1 ] <=> $a->[ 1 ] }
  map [ $_, (/(\d+)/)[ 0 ] ],
  @files;

__END__
[download]