use XML::Rules;

my @numbers;
my $parser = XML::Rules->new(
 rules => [
  '_default' => '', # not interested in most tags
  'CS_REFCLT' => sub {push @numbers, $_[1]->{_content}; return},
 ],
);

$parser->parse($filename);
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This way you don't have to worry whether there's just one <CS_REFCLT> on a line or whether there are more, etc.