#!/usr/bin/perl

use strict;
use warnings;
use Tie::File;

# my $pattern = qr/(^4000.+)/;
# my $pattern = qr/(^4001.+)/;
my $pattern = qr/(^4002.+)/;
# my $pattern = qr/(4003.+)/;
# my $pattern = qr/(^4004.+)/;
# my $pattern = qr/(^4005.+)/;

tie my @lines, 'Tie::File', shift || die;

my $idx = 0;

for my $line(@lines){
   print qq($idx $line\n);
   if( $line =~ m/($pattern)/ ){
      if( $idx == 0){
         print qq(Heuraka: $1 next: $lines[ ( $idx + 1) ]\n);
      };
      if ( $idx == scalar( @lines - 1 ) ) {
          print qq(Heuraka: $1 previous: $lines[ ( $idx - 1 )] \n);
      };
      if ( $idx ~~ [ 1..scalar( @lines - 2 ) ]) {
          print qq(Heuraka: $1 previous: $lines[ ( $idx - 1 ) ] next: $lines[ ( $idx + 1 ) ]\n);
      };
    }
    ++$idx;
}

untie @lines || die;

__END__

Karls-Mac-mini:Desktop karl$ cat MyData.txt
4000_1#0
4001_1#1
4002_1#2
4003_1#3
4004_1#4
4005_1#5

Karls-Mac-mini:Desktop karl$ ./test.pl  MyData.txt 
0 4000_1#0
1 4001_1#1
2 4002_1#2
Heuraka: 4002_1#2 previous: 4001_1#1 next: 4003_1#3
3 4003_1#3
4 4004_1#4
5 4005_1#5