You could start by reading the documentation. The first place to start looking would be perlre.

Your question is a little ambiguous, because you say "all-cap letters" and "no numbers". What of the rest of the character set that is neither a letter (uppercase or not) or a digit?... Spaces, punctuation, control characters and so forth.

Be that as it may, a nudge in the right direction would be something like:

  if( $line =~ /^[A-Z]+$/ ) {
    # matched
  }
  else {
    # did not match
  }
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You may have to amend the character class in order to achieve exactly what you want.