Mine does replace the slash when it has a digit on only one side. The logic is that the slash is replaced unless there is a digit on both sides. It is the OP's logic that will not replace a slash with a digit on either side.

for my $field ( map $_, 'C/O', '2/4', 'C/2', 'd/l', '2/', '/2') {
  print "$field...";
  $field =~ s/
    (?<!C(?=.O))   # Look back a character and make sure it's not C-so
+mething-O from there
    (?<!\d(?=.\d)) # Make sure it's also not a digit-something-digit
    (?:\\|\/)      # The something is a slash or backslash
  / & /xgi;   
  print "$field\n";
}
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