Well -- to you, talk about ungrateful. It would be better to state more clearly what you want up front.

Adding this additional constraint changes the pattern but a little. Were you to study perlre, you could probably have done it yourself.

sub is_valid {
    my $test = shift;
    return $test =~ /^aaa_\w{1,24}(?<!_)_\d{4}$/;
}
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