You could accomplish that by using "soft" references but it is not recommended to use them since they pollute your namespaces.
Instead, use a hash; e.g.,

my %vars = (
  'login' => undef,
  'campaign' => undef,
  'work' => undef,
);
foreach my $func ('login','campaign','work') {
  my $tmp = calc_difference($var1, $var2);
  $vars{$func}{tot} = $tmp;
  # Or just append 'tot' to the key, e.g.,
  # $vars{"${func}_tot"} = $tmp;
}
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--perlplexer