Regex: Named Captures 

• Using named captures:

while(<>) {
    m{
        ^\S+                                  # Client host/IP
        \s\S+                                 # Identd user
        \s\S+                                 # HTTP auth user
        \s\[.*?\]                             # Date/timestamp
        \s"GET\s(?<REQ_URL>/\S*)\s[^"]*"      # URL path requested
        \s\d+                                 # HTTP status code
        \s\d+                                 # Bytes sent in body
        \s"\Q$site_root\E(?<REF_URL>/[^"]*)"  # Referring URL path
    }x or next;
    $tally{ "$+{REF_URL} => $+{REQ_URL}" }++;
}