This minor modification worked for me:

eval {
        no strict "refs";

        require "$fqn_file";

        local $SIG{'__DIE__'};
        ###$ref = $foo->can('f_preferred') ? \&{$foo . "::f_preferred"
+} : undef;
        $ref = $foo->can('f_preferred');
};
[download]

Read up on UNIVERSAL for more info.

Update: I too needed to read up on UNIVERSAL, as can does exactly what the OP was looking for without my redundant ternary operator.