${$var} means
"the scalar variable referenced by the reference in $var." or
"the scalar package variable referenced by the name in $var."

my $some_scalar = 'abc';
my $ref = \$some_scalar;
print $ref, "\n";      # SCALAR(0x225e9c)
print ${$ref}, "\n";   # abc
[download]

$some_scalar = 'abc';
my $name = 'some_scalar';
print $name, "\n";     # some_scalar
print ${$name}, "\n";  # abc
[download]

See perlref for the former, and Why it's stupid to "use a variable as a variable name" for the latter.