Here's my go at it. I did a compare file and it looks like this produces an identical output to your code. 
#! perl -slw
use strict;
use List::Util qw[ reduce ];
use Data::Dump qw[ pp ];

printf "%s", scalar <DATA>;
my @points = map[ split ' ', $_ ], <DATA>;

my $i=0;
while ($i < @points-1)
{   
   my ($x1,$y1) = @{$points[$i]};
   my ($x2,$y2) = @{$points[$i+1]};
   last unless defined $x2;
   
   if ( ($y2 - $y1 ) / ( $x2 - $x1 ) < 0 )
   {
      splice @points,$i,2; #remove both points
      $i=-2;               #backup an extra point to restart
      $i=0 if $i<0;
   }
   $i++;
}
print join ' ', @$_ for @points;
 
__DATA__
X Y
0.0036759    0.4018006
0.0036962    0.4074616
0.0037064    0.4124646
0.0036962    0.4170003
0.0037064    0.4216399
0.0037166    0.4351084
0.0037064    0.4396787
0.0037166    0.4438854
0.0037268    0.4518142
0.0037166    0.4593275
0.0037268    0.4628417
0.0037370    0.4730210
0.0037268    0.4764141
0.0037370    0.4832176
0.0037268    0.4864203
0.0037370    0.4894152
0.0037471    0.4952320
0.0037675    0.4979326
0.0037879    0.5014988
0.0038082    0.5057747
0.0038184    0.5166984
0.0038286    0.5332830
0.0038184    0.5387016
0.0038082    0.5500408
0.0038184    0.5558402
0.0038286    0.5613627
0.0038388    0.6026338
0.0038591    0.6216075
0.0038490    0.6257104
0.0038693    0.6343489
0.0038490    0.6382094
0.0038693    0.6420872
0.0038490    0.6455669
0.0038693    0.6536515
0.0038591    0.6673797
0.0038693    0.6709805
0.0038795    0.6808655
0.0038897    0.6866130
0.0039202    0.6981425
0.0039406    0.7057251
0.0039610    0.7105550
0.0039508    0.7161121
0.0039610    0.7216518
0.0039712    0.7329564
0.0039610    0.7381326
0.0039712    0.7433434
0.0039813    0.7482426
0.0039712    0.7529860
0.0039813    0.7577987
0.0039712    0.7622998
0.0039915    0.7713018
0.0040119    0.7911064
0.0039915    0.7945342
0.0040017    0.7981869
0.0040119    0.8014242
0.0040221    0.8074314
0.0040119    0.8101147
0.0040221    0.8130404
0.0040119    0.8156025
0.0040221    0.8210730
0.0040119    0.8237044
0.0040221    0.8264223
0.0040526    0.8290191
0.0040628    0.8323083
0.0040933    0.8361688
0.0041035    0.8409814
0.0041239    0.8466942
0.0041137    0.8527880
0.0041239    0.8591068
0.0041341    0.864785
0.0041239    0.8703767
0.0041443    0.8760895
0.0041544    0.8858707
0.0041646    0.8995296
0.0041748    0.9034420
0.0041646    0.9072853
0.0041748    0.9111804
0.0041646    0.9148159
0.0041748    0.9187110
0.0041850    0.9221387
0.0041748    0.9254799
0.0041850    0.9288903
0.0041748    0.9322834
0.0041850    0.9357977
0.0041952    0.9453537

[download]
Output: 
X Y
0.0036759 0.4018006
0.0036962 0.4074616
0.0037064 0.4216399
0.0037166 0.4438854
0.0037268 0.4628417
0.0037370 0.4894152
0.0037471 0.4952320
0.0037675 0.4979326
0.0037879 0.5014988
0.0038082 0.5057747
0.0038184 0.5558402
0.0038286 0.5613627
0.0038388 0.6026338
0.0038693 0.6709805
0.0038795 0.6808655
0.0038897 0.6866130
0.0039202 0.6981425
0.0039406 0.7057251
0.0039610 0.7216518
0.0039712 0.7433434
0.0039915 0.7713018
0.0040017 0.7981869
0.0040119 0.8014242
0.0040221 0.8264223
0.0040526 0.8290191
0.0040628 0.8323083
0.0040933 0.8361688
0.0041035 0.8409814
0.0041239 0.8591068
0.0041443 0.8760895
0.0041544 0.8858707
0.0041646 0.8995296
0.0041748 0.9187110
0.0041850 0.9357977
0.0041952 0.9453537

[download]

In reply to Re: How would you code this? by Marshall
in thread How would you code this? by BrowserUk

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