First off, you almost certainly want to use a hash instead of doing this. Have a read of Why it's stupid to `use a variable as a variable name', A More Direct Explanation of the Problem and What if I'm Really Careful?.

For example do this:

my %totals;
foreach my $func ('login','campaign','work') {
     my $tmp = calc_difference($var1, $var2);
    $total{$func) += $tmp;
 }
[download]

If you really need to do it do a super search on symbolic references :-)