You can write

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

also as

([a-z0-9])+
[download]

or

[a-z0-9]+
[download]

which will save you a lot of capturing clauses if you don't need to capture anything

I didn't analyze the whole regex, but at a first view some questions arised:

x) what's about _ and uppercase letters? -> \w instead of a-z0-9
x) \@{1} is the same as \@ => so {1} can be omitted, also in \.{1}
x) what about user@my-domain.tld ?
x) x.@domain.tld seems ok

BTW: with the flag /x you can write regexes more readable and comment them, e.g.

$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;
[download]

for scripts in production I like to use Mail::RFC822::Address from CPAN...