With the line my $textIn = <STDIN>; you only get one line of input. If that line has no backticks you get no substitution. You probably want to do a while (<STDIN>) {} loop around the substitution and printing.

If you get a line with two backticked sections, you will get a surprise. The regex qr/`(.*)`/ is greedy and will match everything between the first on the line and the last. Try qr/`(.*?)`/ or qr/`([^`]*)`/ to get what you want.