Even better, a version that expands ':' deeply (Update:) AND handles !:tag...

package MyExporter;

use Exporter ();

BEGIN { our @ISA = 'Exporter'; }

sub import {
   our @EXPORT;
   our @EXPORT_OK;
   our @EXPORT_TAGS;

  #my $pkg = \%::;
  #$pkg = $pkg->{$_} foreach ((caller().'::') =~ /\G(.*::)/g);

   my $pkg = caller() . '::';
   { no strict 'refs'; $pkg = *$pkg; }

   local *EXPORT      = $pkg->{'EXPORT'};
   local *EXPORT_OK   = $pkg->{'EXPORT_OK'};
   local *EXPORT_TAGS = $pkg->{'EXPORT_TAGS'};

   $EXPORT_TAGS{'all'} = [ @EXPORT, @EXPORT_OK, @EXPORT_TAGS ];

   {
      # Expand :tag into components, deeply.
      # Cycles are silently broken in an arbitrary manner.

      my %expanded;

      my $expand = sub {
         my ($key) = @_;
         return if ($expanded{$key}++);
         my %list;
         foreach (@{$EXPORT_TAGS{$key}}) {
            if (/^!(:.*)/) {
               my $tag = $1;
               &$expand($tag);
               delete(@list{@{$EXPORT_TAGS{$tag}}});
            } elsif (/^:.*/) {
               my $tag = $_;
               &$expand($tag);
               %list = (%list, map { $_ => 1 } @{$EXPORT_TAGS{$tag}});
            } else {
               $list{$_} = 1;
            }
         }
         @{$EXPORT_TAGS{$key}} = keys(%list);
      };

      &$expand($_) foreach (keys(%EXPORT_TAGS));
   }

   &Exporter::import;
}
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(untested)