Module::Runtime is probably what you want to use for this. Here are the relevant parts...

sub _is_string($) {
   my($arg) = @_;
   return defined($arg) && ref(\$arg) eq "SCALAR";
}

our $module_name_rx = qr/[A-Z_a-z][0-9A-Z_a-z]*(?:::[0-9A-Z_a-z]+)*/;

sub is_module_name($) {
   _is_string($_[0]) && $_[0] =~ /\A$module_name_rx\z/o;
}
[download]

This is more restrictive than your current check - in particular it excludes all non-ASCII word characters. This is because Unicode file names are handled pretty inconsistently across different file systems and Perl versions.

Note that this is really a module name check; not a package name check. Modules are files on the filesystem; packages are namespaces. Package names are a lot more relaxed than module names; for example, the space character can be used as a package name:

perl -E'*{" ::foo"} = sub {42}; say " "->foo'
[download]