my @files = qw(3_xy.txt 10_foo.txt 2_bar.txt 1_foobar.txt);

@files =
	map  { $_->[0] }
	sort { $b->[0] <=> $a->[0] }
	map  { [split(/_/, $_)]  } @files;

foreach my $file (@files){
	print "$file\n";
}