You may want to use hash in this case.
Store functions as 'keys' and totals as 'values'

my %total;
foreach my $function (qw(login campaign work)){
    # do some things
    my $tmp = calc_difference($var1, $var2);
    $total{$function} += $tmp;
}
foreach my $function ( keys %total){
    print "$function =>",$total{$function},"\n";
}
#Single function
print $total{login};
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