PackageName->can('sub_name');

##</code><code>##

$PAGE = "test_function_a";
eval "$PAGE" if main->can($PAGE);

##</code><code>##

my %dispatch = (
   'test_function_a' => \&test_function_a,
   'test_function_b' => \&test_function_b,
);

foreach my $func ( qw'test_function_a test_function_b' ) {
   die "Package main doesn't seem able to '$func'" unless main->can($func);
   $dispatch{$func}->();
}