I wrote a module to generate such regular expressions, but it's not yet available on CPAN. You can find it currently on Github at Regexp-PasswordValidation.

The approach of the regular expressions it creates is to generate one lookahead group for each required character from a set:

(?=(?:.*[A-Za-z].*){1}) # at least one of A-Za-z
(?=(?:.*[0-9].*){1})    # at least one of 0-9
(?=(?:^.{8}$)           # match exactly 8 charactters
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... and so on.

Combining these gives:

qr/^
(?=(?:.*[A-Za-z].*){1}) # at least one of A-Za-z
(?=(?:.*[0-9].*){1})    # at least one of 0-9
(?=(?:^.{8}$)           # match exactly 8 charactters
$
/x;
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