sub make{

  my ($self,$arg) = @_;

  if (ref $arg eq 'ARRAY'){
    foreach my $t (@{$arg}){
      if (ref $t eq 'HASH'){
        _ss($t)
      };
    }
  }

}

sub _ss {
  my ($self, %arg) = @_;

  print Dumper(\%arg);
}
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You probably mean

        $self->_ss($t)
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since in sub _ss() you are shifting away the first argument into $self.
What happens next? You assign the arguments to a hash, but in the calling code you just pass a single reference, not a key/value pair. So your hash reference ends up as a stringified key in %arg with no value attached.