I'd recommend against using prototypes except where neccessary in Perl. They are not equivalent to C/C++ formal parameters.

But if you must, you can do it this way:

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 (e
+val 7) line 1, at EOF
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Note the ';' between the prototype args which make everything after it optional.

Note also the error generated by the last test above. This is just one of several limitations of Perl prototypes.