my %dist_0_00;   # for exact matches
my %dist_0_05;   # for points that may match within 0.05
my %dist_0_50;   # for points that may match within 0.50
my %dist_5_00;   # for points that may match within 5.00
my ($y005,$y050,$y500);

for $y (@y_set) {
   $y005 = floor($y*10)/10;
   $y050 = floor($y);
   $y500 = floor($y/10)*10;
   $dist_0_00{$y}=[$y];
   push @{$dist_0_05{$y005}},     $y;
   push @{$dist_0_05{$y005+0.1}}, $y;
   push @{$dist_0_50{$y050}},     $y;
   push @{$dist_0_50{$y050+1.0}}, $y;
   push @{$dist_5_00{$y500}},     $y;
   push @{$dist_5_00{$y500+10.0}},$y;
}
# so if $x = 18.94, all the matches that are within 0.05
# of it are in either in the array at arrayref
# $dist_0_05{18.8} or in the one at $dist_0_05{18.9} 
# similarly, the matches within 0.5 are in two hash elements
# of %dist_0_50.

my @matches;
my $x_bin;
for $x ($x_set) {
   if ( $dist_0_00{$x} ) {
      # exact match
      push @match,[$x,$x];
      next;
   }
   $x_bin = floor(($x-0.05)*10).10.0;
   if (   @y_test_low = $dist_0_05{$xbin}
       || @y_test_high= $dist_0_05{$xbin+0.1} ) {
      if (defined($y=have_best_within(0.05,@y_test_low,@y_test_high)){
         push @match,[$x,$];
         next;
      }
   }
   # similarly for 0.5 and 5.0 thresholds
} # for $x