Let me reformat your code a little bit in order to make its purpose clear:

#!/usr/bin/perl -w
use strict;
my %myhash=();
my $myhash;

open(DATA_LIST, "<data_list") or die "Can't open data_list to read!: $
+!\n";
while (<DATA_LIST>){
  chomp;
  my ($Line,$Filed,$Dvalue)=split/\t/;
  #open(MYHASH, ">myhash") or die "Can't open myhash to write : $!\n";

  # vladb: reading data from file 'file_data' into %myhash hash..
  do_hash("file_data");

  # vladb: are you absolutely sure value stored in $Line
  # was also found in the 'file_data' file which was used to
  # initialize the %myhash hash structure?
  print $myhash{$Line},"\n";  # *** vladb: move this line inside the f
+ollowing if(..){} block.

  if (exists $myhash{$Line}){
    print $Filed,"\t ",$Dvalue,"\t", $myhash{$Line},"\n";
  }
} 

sub do_hash {
  my ($filename)=@_;
  open(FH, $filename) or die "Can't open $filename: $!\n";
  while(<FH>){
    chomp;
    my ($Name, $Data)=split/\t/;
    $myhash{$Name}=$Data;
  }
  close FH;
}
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Please refer to my comments inside your code. The problem here, essentially, is that you are reading records from 'file_data' file and storing them in a global %myhash hash structure such that the first field of each record is your hash key, while the second field is your value. However, as you read records (lines) from the 'data_list' file and use the first field value (stored in the $Line) variable to reference a value in the %myhash hash, there's no absolute guarantee that the hash contains a matching key in the first place. One suggestion would be to move the print $myhash{$Line},"\n"; line inside the if block that follows. This way, you'll never attempt to access a hash record via a non-existant key.

UPDATE: Dear, Anonymous Monk you have to absolutely check for a key to exist in a hash before actually using it to retrieve any data from that hash. Therefore, to avoid the kind of error you were getting, my suggestion would be to move the line marked with '***' inside your if block where you check for the key to 'exist' before using it...

_____________________

$"=q;grep;;$,=q"grep";for(`find . -name ".saves*~"`){s;$/;;;/(.*-(\d+)
+-.*)$/;$_=&#91"ps -e -o pid | "," $2 | "," -v "," "]`@$_`?{print"
++ $1"}:{print"- $1"}&&`rm $1`;print"\n";}
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