You have it almost right, but Perl needs to know which package things are coming from. Here's one way to do it:

use strict;

package my_temp_convert;

my $cur_ctemp=0;

sub set_ctemp {$cur_ctemp = shift;}
sub c {return $cur_ctemp;}
sub f {return 32 + (9 * $cur_ctemp) / 5;}

package main;

my_temp_convert::set_ctemp(32);
printf "%d F is %d C\n", my_temp_convert::f(), my_temp_convert::c();
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Prints:

89 F is 32 C
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