I assume what's giving you trouble is the:

$answer = $pcr * $cell_size * 8;
$len1 = length($answer);
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part (which computes a number and then calculates the length of its stringified form). Perl by default uses something like $str = sprintf("%.15g",$num) to stringify floating point numbers (where the %g format chooses whether to look like %e or %f). You can do it manually instead, never using the scientfic notation, with:

$answer = sprintf "%.0f", $answer;
$len1 = length($answer);
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You might be better off checking for numberic ranges; for instance (untested):

if ($answer >= 1000 && $answer < 1000000) {
   $answer = int($answer/1000) . " Kb/s";
} else ...
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or a version that incorporates rounding instead of truncation:

if ($answer >= 999.5 && $answer < 999500) {
   $answer = int($answer/1000 + .5) . " Kb/s";
} else ...
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