There are many ways of doing what you want. Here is the quickest one, assuming that you don't need a return value on success:

sub add_user {
 if (condition) {
  return 'Oops';
 } else {
  return 'Argh';
 }
 return undef;
}

my $Result = &add_user;
if ($Result) {
 print "Something failed: $Result\n";
}
[download]

This also works if you need a return value, making things a very little bit more complicated:

sub add_user {
 if (condition) {
  return 0,'Oops';
 } else {
  return 0,'Argh';
 }
 return 1;
}

my ($Result, $ErrorText) = &add_user;
if ( ! $Result) {
 print "Something failed: $Error_Text\n";
}
[download]

Here is the multilanguage style which is also good whenever you want a program to parse your result (parsing text error messages is ugly):

sub add_user {
 if (condition) {
  return 1;
 } else {
  return 2;
 }
 return 0;
}

my $Result = &add_user;
if ($Result) {
 print "Something failed: ".$Texts{'Error_'.$Result}."\n";
}
[download]

Finally, there is an OOP approch where you define a package and bless it on a hash where the error message and other things could be stored, but this is way too big for your problem...