Why don't you forget Fahrenheit and Celsius and use the Kelvin scale instead? After all it does place absolute zero at zero and work upwards. You can use a conversion function to change the input to Kelvin and test for the start temp being greater than zero and less than the end temp.

Using Cody Pendants post as inspiration. Implementing the Kelvin / Fahrenheit/Celsius conversion functions and better presentation is left as an exercise for the reader...

#! /usr/bin/perl
use strict;
use warnings;

kelvin (@ARGV);

sub kelvin
{
    my $start = $_[0];
    my $end = $_[1];
    my $inc = $_[2];
    
    if (($start > 0) and ($start < $end))
    {
        print "Fahrenheit Celcius\n";
        my $loop = $start;
        while ($loop < $end)
        {
            print $loop, "\n";
            $loop = $loop + $inc;
        }
    }
    else
    {
        die ("You entered bad arguments dumbass!");
    }
}
[download]

Or just mess with his head...

k2 (@ARGV);

sub k2 {s{}{$_[0]-$_[2]}e;while(
$_[0]>=0&&$_[0]<$_[1]&&s>([\d.]+)>
$1<$_[1]?$1+$_[2]:''>e){print+qq
+$_\n+};!$_||die ("Bad Params!")}
[download]