You need to either use the "x" modifier for your match (so that it ignores whitespace in the pattern) or remove the space between your first pattern and the |. Also, you're requiring whitespace before the date, but there is none in your data. This will work:

my $pattern = '(\s*\w+\s+\d{2}\s\d{4}) | (Release\s+\d+\.\d+)';
while(<LOG>) {
    print $1  if($_ =~ /($pattern)/x );
}
[download]

and so will this:

my $pattern = '(\s*\w+\s+\d{2}\s\d{4})|(Release\s+\d+\.\d+)';
while(<LOG>) {
    print $1  if($_ =~ /($pattern)/ );
}
[download]