A common way to do this is to provide a method in your module that will return the last error that occured. Something like:

...
   my $age = Validate->numbers ($query->param('age'));
   push @errors, Validate->lasterr unless $age;
...

package Validate;

our $lasterr;

sub numbers {
   my ($class, $value) = @_;
   if ($value
   $lasterr = "Please, use only numbers.";
   return undef unless $value =~ /^([0-9 \.-]*)$/;
   return "$1";
}

sub lasterr {
  return $lasterr;
}

1;
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