If any in-scope variable value contains a literal carriage return (\r) character, the display in the Debugger Variables window is corrupted. The variable with the embedded carriage return is hidden, and some unrelated variables in the display may also be hidden.
If, later in the program execution, the embedded carriage return disappears from the variable value, the Variables window display returns to normal.
Single-stepping through the following simple test case reliably reproduces the bug; here the variable $var1 is hidden when its value contains a carriage return, and so is the unrelated variable $var2:
my $var1 = "This is a variable.";
my $var2 = "This is an unrelated variable.";
$var1 = "A carriage return \r is embedded.";
$var1 = "No more carriage return!";