my @ambiguous;
BORKEN:
while (<$fh_b0rken>) {

    my @b0rken;   # can use with perl version 5.18+
  # local our @b0rken;  use re 'eval';  # pre-5.18 version
    $_ =~ m{
        \A (\s* CMD \s* = \s* " $rx_reference) ((?! /) [^\n]*) \n? \z
        (?{ push @b0rken, [ $1, $2 ] }) (*FAIL)  # (?!) pre-5.10
        }xms;

    if (@b0rken > 1) {  # more than 1 possible breakage: report ambiguity
        chomp;
        push @ambiguous, join "\n",
             "'$_' ambiguous:", map "  '$_->[0]'?'$_->[1]'", @b0rken;
        }
    elsif (@b0rken == 1) {  # just 1 breakage: fix and output
        print "$b0rken[0]->[0]$add$b0rken[0]->[1]\n";
        }
    else {  # path looks ok: just output
        print;
        }

    }  # end while BORKEN

...

warn $_ for @ambiguous;  # report ambiguities