You should always declare variables with either our or my.

"whichever package statement is in effect".Here's how you set the package.

use strict;
use warnings;

# the default namespace is main
# so this is a variable in main.
our $movie='Chitty Chitty Bang Bang';

# set the current package
package A::B::C;

# We've changed the package - it is no longer main
# now its A::B::C

# if a variable is undefined in the current package,
# then Perl looks for a variable in main of the same name
# these two statements are the same and do not cause
# warnings

print "movie=$movie\n";
print "movie=$main::movie\n";

# but once we declare the variable in A::B::C, an
# unqualified name is presumed to be in the current 
# package

# declare a package variable in the namespace A::B::C
our $movie = 'Mary Poppins';

# these print different things
print "movie=$movie\n";       # prints Marry Poppins
print "movie=$main::movie\n"; # prints Chitty Chitty Bang Bang

{
   # this overrides the current package. The override
   # will last until the next matching curly brace
   package NotA::NotB;

   # this is $movie but in the NotA::NotB namespace
   # its a different variable from either $main::movie
   # or $A::B::C::movie

   my $movie = 'Pippi Longstocking';
   
   print "movie=$movie\n";          #prints Pippi Longstocking
   print "movie=$A::B::C::movie\n"; #prints Mary Poppins
   print "movie=$main::movie\n";    #prints Chitty Chitty Bang Bang

# } matches { enclosing package statement
# so this ends the code block where NotA::NotB is the
# package.
}

# now we are back to A::B::C being the current package 
print "movie=$movie\n";       # prints Marry Poppins
print "movie=$main::movie\n"; # prints Chitty Chitty Bang Bang
[download]

Hope that helps.

Best, beth

So if I declare a variable without "my" , is that a package variable ?

There are three ways to declare a variable.

my $var; declares a lexical var.
our $var; declares a package var. The declaration is lexical.
use vars qw( $var ); declares a package var. The declaration is global.

If I declare a variable in the sub routine without "my" , is it still a package variable ?

It's irrelevant whether a sub is being compiled or not.

How to know which package statement is in effect ?

Look for a package statement in the source, or call __PACKAGE__.

If I want to use same variable name in another program, how can I do that ?

The question makes no sense. Please clarify