It looks like you are trying to use a negated character class ([^ ... ]) to enclose your alternation of words you don't want. That will not work as character classes just deal with single characters. Perhaps something like this would be better.

my $forbidden = qr{^(?:master|model|dbccdb|sybsecurity|sybsystemdb|syb
+systemprocs|tempdb|DBA)[a-zA-Z0-9_]+};
if ( $entered =~ $forbidden )
{
    # Your rejection code here
}
[download]

I hope this is helpful.