I'm assuming the line can contain characters that aren't letters, and that you just want to make sure there are no lowercase letters or numbers: 
while (<DATA>) { print if $_ !~ /[[:lower:]\d]/; }

__DATA__
THIS LINE IS ALL CAPS
THIS LINE IS CAPS WITH 012
THIS LINE has lowercase
THIS LINE has lowercase and 012

[download]
(if he'd meant that the line should only contain capital letters, he wouldn't have mentioned the no numbers part)

In reply to Re: Recognizing all-caps line by TedPride
in thread Recognizing all-caps line by ParisR

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