$line =~ m/(?:      # Collect heading fields
               [^;]+  # delimited by semicolons
               ;
             ){1}     # In your case, there is only one field before the interesting field
             =        # followed by a "="
             (\d+)    # and followed by our numbers, stuff them into $1
             ;        # and delimited by another semicolon
            /x;