my $least;
print map { abs($_->[0]-$least)<500 && $_->[1] }
      sort{ $a->[0] <=> $b->[0] }
      map { /(\d+)$/;
            $least = $1 if !defined $least or $1<$least;
            [ $1, $_]
          } <DATA>
  
__DATA__
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
# prints:
ID4 ch1 120 mir abc xyz ch2 2025
ID3 ch1 100 mir abc xyz ch2 2045
ID1 ch1 70 mir abc xyz ch2 2050