use Symbol qw( gensym );

my $old;
{
    no strict 'refs';
    $old = \*{ "package::func_name" };
}

my $new = gensym;
*$new = *$old{$_} foreach ( grep { defined *$old{$_} } qw( SCALAR ARRAY HASH IO FORMAT ) );

{
    no strict 'refs';
    *{ "package::func_name" } = *$new;
}