Are "old-school" lexical subs an option? That is:

my $looks_like_number = \&Scalar::Util::looks_like_number;
...
... $looks_like_number->( $x ) ...

I personally like this approach because it gives me an easy out if the function is unavailable:

my $looks_like_number = Scalar::Util->can( 'looks_like_number' ) || sub { ... };