Originally there were only singles ticks around the regular expression and I changed them to double quotes because I thought perl was interpreting the $d as a variable but that was not the case.

It's neither in single nor double quotes. It's in backticks. The contents of backticks are subject to interpolation. (After which they are executed as a shell command and their output is returned. See perlop) If you're trying to pass a literal dollar sign, escape it with a backslash.