Grab the CODE slot of the typeglob if you want to check if a function is defined. The existance of any of the package variables $f_preferred, @f_preferred, %f_preferred, etc will cause exists to return true, so exists is not suitable.

my $pkg      = ...;
my $mod_file = ...;

# require always returns true or an exception.
eval { require $mod_file }
   or warn $@;

my $sym = do { no strict 'refs';  ${ "${pkg}::" }{ 'f_preferred' } };
my $ref = *$sym{CODE};
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If the function hasn't been declared, or if the function has been declared (sub func;) but not defined (sub func { ... }), then $ref will be undef.

Update: Rephrased for increased clarity. Code unchanged.