What about using can...

eval do { local $/; <DATA> }; # psuedo require!
die $@ if $@;

for my $sub ( 'foo', 'undefined' ) {
    if ( main->can($sub) ) {
        main->$sub;
    }
    else {
        print "$sub() not defined\n";
    }
}

__DATA__
sub foo {
    print "foo() worked\n";
}
[download]