That is called Symbolic reference and is illegal under use strict pragma. You have to temporary disable use strict and enable it again after the print statement.
#!/usr/bin/perl
@a = (1, 2, 3);
@b = (4, 5, 6);
@c = (7, 8, 9);
$x = 'b';
no strict 'refs';
print "@$x\n";
use strict 'refs';