my $string = 'Aug 20, 2007 12:42 PM';
if ($string =~ /(\w{3}) (\d+), (\d{4}) (\d+):(\d{2}) (.+)/) {
   # do stuff
}

##</code><code>##

if ($string =~ /(\w{3}) (\d+), (\d{4}) ((\d+):(\d{2}) (.+)?)/) {
   # do stuff
}