G'day Chris Clarke,

Welcome to the monastery.

"Any idea how I can deal with these as I am unable to define the field name as a variable with all the $s in it?"

Without seeing your code, I'm somewhat guessing here; however, is there some reason you can't simply code:

my $uname_field_name = 'ctl00$ContentPlaceHolder1$Login1$UserName';
my $pword_field_name = 'ctl00$ContentPlaceHolder1$Login1$Password';
[download]

Take a look at perlop - Quote-Like Operators and quotemeta.

Given that my solution is so straightforward (and, to me, very obvious), I suspect there may be something else going on here. If there is, please take a look at "How do I post a question effectively?" to get a better answer.