There are two basic approaches you can take here

1. Capture all words that fit your requirement and return that list
2. Remove all words that do not fit your criterion, and return the result

In general, it is much easier to write positive regexes than negative ones, so I would use the second approach. I would do something like:

#!/usr/bin/perl
use strict;
use warnings;

my $data = "Antler embedded in mound at South Street, Avebury, Wiltshi
+re, England. Comment (lab): Collagen fraction used";

my @result;
while ($data =~ /\b([A-Z][a-z]*)/g) {
    push @result, $1;
}
print join(' ', @result), "\n";
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YAPE::Regex::Explain explains this as

The regular expression:

(?-imsx:\b([A-Z][a-z]*))

matches as follows:
  
NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  \b                       the boundary between a word char (\w) and
                           something that is not a word char
----------------------------------------------------------------------
  (                        group and capture to \1:
----------------------------------------------------------------------
    [A-Z]                    any character of: 'A' to 'Z'
----------------------------------------------------------------------
    [a-z]*                   any character of: 'a' to 'z' (0 or more
                             times (matching the most amount
                             possible))
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------
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See perlretut for more details.