Use a complemented transliteration in scalar or boolean context: 
if ( $password =~ tr/A-Za-z0-9//c ) {
    warn "Bad characters in <$password>\n";
}

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The question of "sensibility" is left to the implementor. :)

Update: AidanLee found an extra right paren which has since been recycled. Oops.

In reply to Re: Yet another REGEXP Question by chromatic
in thread Yet another REGEXP Question by fastkeys

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