This may seem to be a very naive approach, but what about simply removing lines where X is going down compared to previous values? Something like this: 
use strict;
use warnings;

my $previous_x = 0;

while (<DATA>) {
    my ($x, $y) = split;
    next if $x < $previous_x;
    $previous_x = $x;
    print;
}

__DATA__
0.0036759    0.4018006
0.0036962    0.4074616
0.0037064    0.4124646
[rest of input data omitted for brevity]

[download]
Output: 
0.0036759    0.4018006
0.0036962    0.4074616
0.0037064    0.4124646
0.0037064    0.4216399
0.0037166    0.4351084
0.0037166    0.4438854
0.0037268    0.4518142
0.0037268    0.4628417
0.0037370    0.4730210
0.0037370    0.4832176
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
0.0038286    0.5613627
0.0038388    0.6026338
0.0038591    0.6216075
0.0038693    0.6343489
0.0038693    0.6420872
0.0038693    0.6536515
0.0038693    0.6709805
0.0038795    0.6808655
0.0038897    0.6866130
0.0039202    0.6981425
0.0039406    0.7057251
0.0039610    0.7105550
0.0039610    0.7216518
0.0039712    0.7329564
0.0039712    0.7433434
0.0039813    0.7482426
0.0039813    0.7577987
0.0039915    0.7713018
0.0040119    0.7911064
0.0040119    0.8014242
0.0040221    0.8074314
0.0040221    0.8130404
0.0040221    0.8210730
0.0040221    0.8264223
0.0040526    0.8290191
0.0040628    0.8323083
0.0040933    0.8361688
0.0041035    0.8409814
0.0041239    0.8466942
0.0041239    0.8591068
0.0041341    0.864785
0.0041443    0.8760895
0.0041544    0.8858707
0.0041646    0.8995296
0.0041748    0.9034420
0.0041748    0.9111804
0.0041748    0.9187110
0.0041850    0.9221387
0.0041850    0.9288903
0.0041850    0.9357977
0.0041952    0.9453537

[download]
You might also change the relevant code line to: 
next if $x <= $previous_x;

[download]
if you want to remove duplicate X values.

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

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