if ($value eq 'A') {
    eval "use Blah::First";
} elsif ($value eq 'B') {
    eval "use Blah::Second";
} else {
    eval "use Blah::Default";
}

##</code><code>##

my $basedir = "/some/base/dir/for/perl/Blah";
require "$basedir/$value";
# or ---
eval "use Blah::$value";