sub verify ( $;\$) { print "Got:'$_'" for @_; }

$s = 'fred';

verify 'this';
Got:'this'

verify 'this', $s;;
Got:'this'
Got:'SCALAR(0x195d844)'

verify 'this', 'that';;
Type of arg 2 to main::verify must be scalar (not constant item) at (eval 7) line 1, at EOF