G'day rkk,

Welcome to the monastery.

You seem to have got off on the wrong foot somewhat. Here's a few tips:

• How do I post a question effectively? — essential reading.
• How (Not) To Ask A Question — this includes the "Do Your Own Work" section that LanX was pointing you to.
• How do I change/delete my post? — it would have been better to update your original post with the code rather than creating two new (identical) reply nodes; this tells you how to do that.

Here's the guts of the code I might have used to tackle this problem:

#!/usr/bin/env perl -l

use strict;
use warnings;

my $first;

print join ' ' => @$_ for
    grep { $_->[-1] - $first <= 500 }
    sort { $b->[-1] <=> $a->[-1] }
    map { $first = $_->[-1] unless $first; $_ }
    map { [ split ] } <DATA>;

__DATA__
ID1 ch1 70 mir abc xyz ch2 2050
ID4 ch1 120 mir abc xyz ch2 2025
ID2 ch1 90 mir abc xyz ch2 4000
ID3 ch1 100 mir abc xyz ch2 2045
[download]

After excluding the ID2 line, your data was already sorted. I've changed the order of the lines under __DATA__ to show that the sort works.

Here's the output:

$ pm_sort_exclude.pl
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]

Update: I left the 500 out of the comparison. Fixed: s/$_->[-1] <= $first/$_->[-1] - $first <= 500/