Operator precedence is the cause of this. || has much higher precedence than 'or' does. Consider the following:

#!perl -l

sub foo {
  print "Foo!";
  return "foo";
}

my $barbar=0 || foo();
print $barbar;

print "---";

my $or    =0 or foo();
print $or;

__END__
Foo!
foo
---
Foo!
0
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What this shows is that $var = $x || $y; is handled as $var=($x || $y); and $var = $x or $y; is handled as ($var=$x) or $y; which is actually a logical operator used as flow control and as such is pretty well equivelent to unless ($var=$x) { $y }. Where this gets interesting is that its a lot nicer to write open my $fh,$mode,$file or die "$file:$!" than it is to write my $fh; unless (open $fh,$mode,$file) { die "$file:$!" }