You can get the top-level symbol tables with grep /::$/, keys %:: but that will not include second level or deeper packages. To get a list of all packages, you will need to recurse on each of those.

Here is a simple one-liner for that:

perl -e 'sub dp ($) {my $base = shift; my @nodes = map $base.$_, grep 
+/::$/, keys %{$base}; @nodes = grep !/^(?:::)?main::main/, @nodes if 
+$base =~ /^(?:::)?main/; print join(" ", @nodes),"\n"; dp($_) for @no
+des}' -e 'dp "::"'
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Note the special handling due to :: and main:: being aliases and that this does not work if strict 'refs' is in effect.