Rather than try to detect, I'd probably do something deterministic like export functions that use the singleton:

package Getopt::Long::Framework;
use vars qw( @EXPORT_OK );
@EXPORT_OK= qw( Foo Bar Baz );

require Getopt::Long::Framework::Object;

my $singleton = Getopt::Long::Framework::Object->new();

sub Foo
{
    $singleton->Foo( @_ );
}

...

sub GetObjectPackage
{
    "Getopt::Long::Framework::Object";
}

sub new
{
    my $class= shift @_;
    $class = $class->GetObjectPacket()
        if $class->can("GetObjectPacket");
    ...
}
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(in part, because I prefer to keep my class methods and object methods in separate 'package's anyway)