use strict;
my @unsorted = (
				'864278_864377',
				'864518_864703',
				'851171_851270',
				'855398_855579',
				'856258_856357',
				'861015_861139',
				'866387_866549',
				'791806_792296',
			   '1088806_1082296',
		);
			

my @sorted = map  { $_->[0] }
          sort { $a->[1] cmp $b->[1] }
          map  { [ $_, ( split(/\_/, $_) )[0] ] }
               @unsorted;

			   
			   print join ("\n",@sorted);