If there already is a package Foo in the running program, then "whatever Foo;" is assumed to mean call to static method whatever() in package Foo. If it's not and use strict is not in effect, the "Foo" is considered a bareword and the subroutine import() in current package is called and the string "Foo" is passed as the first parameter. In this particular case the module File::Basename is loaded by CPAN.pm or one of its dependencies, so Perl assumes you meant the first meaning.

It's preferable to use Foo->import() is you mean the static method call and import("Foo") if you mean the subroutine call.