I saw a few problems. Check out perltie for more details as to how this all works.

The constructor seems fine...

package Celcius;

sub TIESCALAR {
     my $class=shift;
     my $data=shift;

     return bless \$data, $class;
}
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Your version of STORE didn't actually STORE the change...

sub STORE{
    my $self=shift;
    my $data=shift;
    $$self = ($data*(9/5)+32);
    }
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And last, FETCH only takes one argument, and it's that one argument you should dereference.

sub FETCH{
    my $self=shift;
    return $$self;
    }

1;
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If you're interested, here's the test code I used to try out my version:

#!/usr/bin/perl -w

use strict;
use Celcius;

my $temp;
tie $temp, 'Celcius';
$temp = 0;
print "Initial temp is $temp (C)\n";

while(<>) {
    chomp;
    $temp = $_;
    print "Temp is $_ (F) ($temp (C))\n";
}
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As an alternative, you could store the Farenheit temp, and do the conversion in FETCH when you return the value.

Oh, and I think it's spelled "Celsius", but I'm not sure :)
--
Mike