In addition to the potential predecence problems with or,

Depends on context. While or is more likely to cause problems within an expression (aside from not being as visually distinctive), or is usually used for flow control because it's more likely to solve precedence problems in that situation. Compare

open my $fh, '<', $qfn
   || die $!;
[download]

with

open my $fh, '<', $qfn
   or die $!;
[download]