...
my $sse = 0;    # sum of squared error ("error" = distance from the average)
while(1)
{
    if ($cnt == 8000) {
        # the amplitude >= 0 <= 1
        ### my $amplitude=$value / $cnt / 255;
        my $mse = $sse / $cnt;      # MSE = mean squared error = average of the sum-of-square-distances
        my $rms = sqrt($mse);       # RMS = root mean squared error = a kind of "average power" measurement for the whole wave
        my $scaled_rms = $rms / 255;    # convert from codes to fraction of fullscale

        # calculated dbFS,rms = 20 * log10($rms)
        my $dbFSrms = 20 * log10($scaled_rms);      # this is the number of dB_RMS from the fullscale of your mic+ADC
        my $dba = $dbFSrms - $DBA_MIC_FS;           # you will need to find out your mic+ADC's fullscale capability, in dBa or dBv or dBmW
        print "Calculated dB(FS)_rms and dB(A)_rms: $dbFSrms dBFS, $dba dBa\n";

        # reset the count and SSE for the next 8k samples
        $cnt = 0;
        $sse = 0;
    }

    my $buffer;

    # read one byte
    read($fh, $buffer, 1);

    if(defined($buffer)) {
        # get an unsigned char
        my $v = unpack("C", $buffer);
#       print "Byte read: $v\n";

        # sum the squared distance from the theoretical mean
        $sse += ($v-127.5)**2;
        $cnt++;
    }
}