You can avoid the use of package globals in a singleton object by using a lexical. For instance:

package My::Singleton;

my $DATA;

sub get_data { 
    my $class = shift;
    $class->_init unless defined $DATA;
    return bless $DATA, $class;
}

sub _init { 
    $DATA = { foo => 42 };
}
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Because $DATA is a lexical ("my") variable, it's only accessable to the subroutines defined in its scope. (In this case, a file scope, but you could add a bare block to hide it from other parts of the same file if you really want to.) See also Class::Singleton.