If I understand your requirements correctly the following sample should do it for you:

#!/usr/bin/perl
use strict;
use warnings;

my $inData = <<IN;
ID1 ch1 70 mir abc xyz ch2 2050
ID2 ch1 90 mir abc xyz ch2 4000
ID3 ch1 100 mir abc xyz ch2 2045
ID4 ch1 120 mir abc xyz ch2 2025
IN

open my $inFile, '<', \$inData;

my @lines = sort {$a->[1] <=> $b->[1]} map {/(\d+)$/; [$_, $1]} <$inFi
+le>;

pop @lines if $lines[-1][1] - $lines[0][1] >= 500;
print for sort map {$_->[0]} @lines;
[download]

Prints:

ID1 ch1 70 mir abc xyz ch2 2050
ID3 ch1 100 mir abc xyz ch2 2045
ID4 ch1 120 mir abc xyz ch2 2025
[download]

The trick is to pair the line text with the sort key (done in the map) then subsequently use the key or the original line as required.