Your errors suggests that provide us the code before this line which may has anything to do with this variables.
Both of the code below works.:

$hashref = {  '$/ 11' => 'a'};
$fieldnum= '$/ 11';
$value = $hashref->{$fieldnum};
print "$value";
[download]

and

$hashref = { a => '$/ 11' };
$fieldnum= 'a';
$value = $hashref->{$fieldnum};
print "$value";
[download]