I'd probably avoid autoload, and just assign the subroutine. Based on what I think you're trying to do, I'd probably have an init function in Base that would setup whatever I might need. Obviously, you'd want to assign '$is_func' to handle your method names relative to the package names.

package Base;
sub init {
    my $class = shift;
    my $is_func = $class.'::is_'.lc($class);
    no strict 'refs';
    *$is_func = sub { return 1 };
}
...
package Foo;
our @ISA = qw( Base );
__PACKAGE__->init();
...
package Bar;
our @ISA = qw( Base );
__PACKAGE__->init();
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