Just a couple of comments. I know this is a sensible issue and I'm concious of it departing of tons of work I put in acquiring a good English accent -to no avail-.
As a native speaker, my perception of mixing up the r and d sounds in pero and raspberry is like an English speaker being surprised by someone mixing up the k and h sounds in kite and ham,
una oclusiva (stop consonant) and a fricative.
Though I'm not the one to lecture about English pronunciation I think it all comes of English pronunciation changing as the language follows the rhythm of stress. I mean, consonants and vocals in the stressed syllable get one pronunciation that change in unstressed syllables, thus, English speakers are used to a shade of values for every sound.
On the other side, Spanish both has a syllabic rhythm and a phonetic alphabet -English should have its own, like Russians have the Cyrillic one-, so spoken language has shaped the written one and vice-versa for centuries, hence we are used to a much narrower palette of sounds for the same consonant or vowel.
The r sound in
pero -a dental flap, more on the side of a trill sound- is analogue to the tt sound in highly colloquial high-speed American English (gotta, in "you gotta be kidding", or better, that a Spanish speaker would imagine to be "iu gora bi kidin" or "berer").
The d sound in raspberry, I admit it may look like Rusty says, similar to the th- sound in
there or
the, but in fact it's still
una oclusiva, and clearly one's tongue is doing almost the same as English speakers do with d and t sounds (pronouncing Spanish d as English th- with resolution certainly sounds like Spanish d, but the tongue is in the wrong place and the effort is unnecessary).
Summarizing. For Spanish d: a stop consonant with the tongue touching the teeth like English d but a tinsy bitsy lower and with less pressure, and not flapping the tongue around the teeth edge like English th-. For Spanish r, sounds similar to popular American English tt, but with the tongue slightly touching above the teeth (
la zona alveolar) -when pressure isn't enough and it doesn't trill at all, it sounds un-Spanish and slightly similar to English th, hence the confusion-.
If anyone wants to practice all the o sounds in Spanish and learn more about that syllabic rhythm as opposed to the rhythm led by stress in English, you may hear the old "rock-murga" song
"Ojo con los Orozco".