main:: is the default. If there are no other packages that's the one your in and there is no need to include the prefix. If there is a package (e.g. a module) that has a variable of the same name then you would need the package prefix to distinguish between the two.

Ok. That means both $main'ora_errstr & $ora_errstr belongs to the same package. Then $main'ora_errstr = $ora_errstr doesn't make any sense.
Actually $main'ora_errstr = $ora_errstr this statement is defined in the subroutine of ora package. This package is being called by many other scripts. Some of the scripts use $ora'ora_errstr & some $main'ora_errstr. I am really confused. Please help me.

I think you have it there. A variable of the same name exists in main and in ora. So you have to spell out which one you mean. Perhaps this will help:

#!/usr/bin/perl

use strict;
use warnings;

# package main (default);

our $ora_errstr = 'main';

{

  package ora;
  
  our $ora_errstr = 'ora';
  
  print "package ora\n";
  print "\$ora_errstr: $ora_errstr\n";
  print "\$main::ora_errstr: $main::ora_errstr\n";
  print "\n\n";

}

package main;

print "package main\n";
print "\$ora_errstr: $ora_errstr\n";
print "\$ora::ora_errstr: $ora::ora_errstr\n";

__DATA__

---------- Capture Output ----------
> "C:\Perl\bin\perl.exe" _new.pl

package ora
$ora_errstr: ora
$main::ora_errstr: main


package main
$ora_errstr: main
$ora::ora_errstr: ora

> Terminated with exit code 0.
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