I am pretty new to Perl. I already love it! I needed to validate what a human operator states it is his email address, and I needed to avoid module installation. I have been tuning this function for a while and it seems to be quite good both in picking out bad formed addresses and in giving very few false positives. I hope it can be useful to someone. I would thank those who can suggest improvements too.
sub es_email_mal_formado {
  my $x = $_[0];
  if ($x =~ m/[^\w\@\.\-]/)    {return "Incluye un carácter prohibido"
+};
  if ($x =~ m/\W\W/)           {return "Tiene dos no alfanuméricos seg
+uidos"};
  if ($x =~ m/\@.+\@/)         {return "Tiene más de una arroba"};
  if (! ($x =~ m/\w\@\w/))     {return "No tiene arroba precedida y se
+guida de alfanumérico"};
  if (! ($x =~ m/\w\.\w/))     {return "No tiene punto precedido y seg
+uido de alfanumérico"};
  if (! ($x =~ m/\@.+\./))     {return "No tiene punto en la parte que
+ sigue a la arroba"};
  if ($x =~ m/^\W/)            {return "Empieza en no alfanumérico"};
  if (! ($x =~ m/\.\w{2,4}$/)) {return "No tiene \"de 2 a 4 alfanuméri
+cos\" al final"};
  if ($x =~ m/(\@|\.).\.\w+$/) {return "Tiene una sola letra antes del
+ último punto"};
  return 0;
}

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In reply to Poor man's email address validation function by Jorge_de_Burgos

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