Maybe this helps:

print "number of vertices: ".$G->vertices."\n";
print "number of edges: ".$G->edges."\n";
print "vertices: ".join (':',$G->vertices)."\n";
print "edges: ".join (':',$G->edges)."\n";

# set attribute name to "A" for edge E1 -> E2
$G->set_attribute("name","E1","E2","A") or die "error setting attribut
+e to edge: $!\n";
print "Name of edge E1->E2 is ".$G->get_attribute("name","E1","E2")."\
+n";
print "Name of edge E2->E3 is ".
      (defined $G->get_attribute("name","E2","E3") ? $G->get_attribute
+("name","E2","E3") : "<undef>") ."\n";
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update: I think you missread the docs concerning has_vertices:

             @V = $G->vertices

       In list context returns the vertices @V of the graph $G.
       In scalar context returns the number of the vertices.

               $G->has_vertices(@v)

       In list context returns a list which contains the vertex
       of the vertices @v if the vertex exists in the graph $G
       and undef if it doesn't.  In scalar context returns the
       number of the existing vertices.
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---- amphiplex