...almost there :)   but you'd still have to get rid of the "::" in case of a package name like "Foo::Module". Also, you could write ref($self) || $self, in which case you could call load_config on both the class or an object instance.

#!/usr/bin/perl

package Base::Module;

sub load_config {
    my $class   = shift;
    my $module  = $class->config_module;
    print "about to require $module\n";
    #require $module;
}

sub config_module {
    $self = shift;
    my $pkg = ref($self) || $self;

    $pkg =~ s#::#/#g;
    # optionally remove "/Module"
    # $pkg =~ s#/[^/]+\z##;

    return "$pkg/Config.pm";
}


package Foo::Module;
use base qw(Base::Module);

sub new {
    return bless {}, shift;
}


package main;

Foo::Module->load_config();
Foo::Module->new()->load_config();

__END__
$ ./727519.pl
about to require Foo/Module/Config.pm
about to require Foo/Module/Config.pm
[download]

BTW, the sub config_module would work the same way when declared in Foo::Module, though - if I'm understanding you correctly - your idea was to put it in the base class...  (Actually, it would first be looked up in Foo::Module, and only then, when this fails, in Base::Module (via inheritance).)