Is there a correlation between where I finish my PhD and where can I secure a Postdoc position I am currently pursuing my PhD in computer science at a mid-rank US institution (ranked 50 in US-news graduate computer science ranking). Assuming I have a decent publication record and good references. Would I have better chances of securing a postdoc position if I applied to openings in the US or would I have better chances if I applied to openings in other countries such as Canada, UK, Australia?
In short: your chances of securing a position is higher with people that know you (or someone you have worked with). 

Whether that is people that are physically around you, or people that are working on the other side of the globe (whom you met in a professional setting e.g. at a conference). This is in line with what @la femme consmique, mentions in the comments, networking is important. 

That aside, I have noticed a general skepticism in the U.S. towards Europe. I mean if you are applying for postdoc positions in the US from a European university, you might have to deal with a certain skepticism that does not exist amongst European institutions. It's almost as if you need to convince them that the university actually exists and you haven't made it up on your own (yes, I am exaggerating a bit).

[I would like to make this very clear that this is a subjective statement from personal experience, not a fact.]