([a-z]|[0-9])+

##</code><code>##

([a-z0-9])+

##</code><code>##

[a-z0-9]+

##</code><code>##

$mail =~ /^        # at the beginning
  [a-zA-z0-9]      # one letter or digit
  [a-zA-Z0-9_\-\.] # opt. letters, digits, underscores.
  \@
  [a-zA-z]         # one letter
  ...
$/x;