Just want to expand a little bit on what pfaut said about $$fieldname.

First let's look at a piece of correct code:

use strict;
$$a = 1;# you cannot say my $$a ;-)
print $$a; #this print out 1;
print $a; #this print out SCALAR(0xnnnnnnnn)
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This is perfectly correct, as I just created a hard reference, which is allowed. The next piece is wrong.

use strict("refs");
my $a = "abc";
my $abc = 1;
print $$a;
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This is wrong, as I attempted to create symbolic reference, which is disallowed when you have strict("refs") turned. As a best practice, symbolic reference should be generally disallowed, as it is the source of many bugs in Perl scripts.