# foo.pl
for (1..2) {
    eval "use bar";
    print "attempt $_: ", ($@ || 'success'), "\n";
}

# bar.pm
eval "require baz";
if ($@) {
  die "bar will be unavailable because baz didn't load properly.";
}
1;

# baz.pm
use missingdependency;

##</code><code>##

# foo.pl
for (1..2) {
    my %saveinc = %INC;
    eval "use bar";
    if ($@) {
        for my $package(keys %INC) {
            delete $INC{$package} if ! exists $saveinc{$package};
        }
    }
    print "attempt $_: ", ($@ || 'success'), "\n";
}