This can be done, but I think it may be saner to go down another route as suggested by Corion and perrin ...

$GD::{$_} = $SVG::GD::{$_}
  for grep !/::$/, keys %SVG::GD::;
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This has a similar effect to the symbol table glob aliasing, but only affects the GD:: symbol table. Again, use with caution as this is even more hackish than the glob alias.