Agreed with stevieb on mocking. My goto for this is Test::Mock::Guard. Basically you are replacing some method on a class or object with your own.

my $guard = mock_guard(
    $original_class,
    {
        GetDataFromOldSystem => sub {...},
    },
);

$obj = $original_class->new(...);
$obj->SomeOtherFunctionThatCalls_GetDataFromOldSystem(...);
[download]

Your provided sub would then return some pre-baked values instead of going to the original system.