my $test="my_variable_name"; #imagine this is a hash key
my $default_value="the value I set"; #while this is the hash value
eval("\$$test = '$default_value';");
print "the value of \$my_variable_name = $my_variable_name\n";
$my_variable_name = "A new value";
print "the value of \$my_variable_name = $my_variable_name\n";