Would there be anything wrong with doing config_module in the base class as: ...

...the only problem is it wouldn't work ;) — at least not with perrin's code snippet.  One thing is that if you use require with a string (as opposed to a bareword), it must be a valid path fragment (i.e. / in place of ::), inlcuding the trailing .pm. The other thing is that when you declare config_module() in the base class, __PACKAGE__ would always be Base::Module, but the point of the exercise was to load different config variants (associated with Foo, Bar, etc.).

I suspected something like that.

I guess my next try would be:

sub config_module {
   $self = shift;
   return ref($self) . '/Config.pm';
}
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And just always calling it as a method?
for(split(" ","tsuJ rehtonA lreP rekcaH")){print reverse . " "}print "\b.\n";

...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
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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).)