I'm actually having difficulty working out in which accent they don't rhyme. Presumably talk is different, but I'm not sure how.
1. The "r" in pork is pronounced in most American accents,
2. The vowel is not the same.
Most of these accents have a rhotic r but not a rhotic (lotic?) l in this position.
Example: "comment and post" for "eat toast".
for example too (for a single word example)
My Canadian ear somehow corrects the "pork" and "talk" so that they still don't quite rhyme (though they're certainly close enough for slant rhyme)
I hear the "r" in pork even if it's less audible than it would be in my English. And the "a" in talk sounds a little more like the "a" I'm familiar with from talk than the "o" in pork does.