Well, the first one's a forehead-smacker: of the strings, the only one that *SATISFIES* your first attempt at a fix (and hence is marked as valid) IS 12345z ("unless the string is five digits followed by one or more non-digits, mark it as invalid")

the second one says "match something that has five digits at the beginning *OR* which contains (somewhere) one or more non-digits. (i.e. the caret doesn't bind the \D to the beginning of the string. You'd have to use parentheses (perhaps memory-free) for that, e.g.:

/^(?:\d{5,}|\D+)/
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I'd suggest changing the logic slightly, and I'll use the nifty ternary operator too:

foreach (@stuff) {
    my $status = ( /^\d{5,}$/ ) ? 
        'valid' : 
        'invalid';
    print "$_ is $status\n";
}
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HTH

Philosophy can be made out of anything. Or less -- Jerry A. Fodor