PriNet:

You can't have arbitrary code in the left of a substitution regular expression. (If you use the e modifier, though, you can do so on the right side.)

So you either need to use interpolation:

$form_feed = chr(12);
$var =~ s/$form_feed//g;
[download]

Or you can just specify the value using a hex or octal literal \x0c or \014:

$var =~ s/\014//g;
[download]

...roboticus

When your only tool is a hammer, all problems look like your thumb.