I'm glad you figured out how to do what you need, but for completeness, I'll add that you can quote the string and access it as a single element (instead of slurping all of @ARGV):

use warnings;
use strict;

if (! defined $ARGV[0]){
    print "Usage: ./script.pl \"multi word quoted string here\"\n";
    exit;
}

my $string = $ARGV[0];

print "$string\n";

__END__

$ ./str.pl "hello, world!"
hello, world!
[download]

Update: I was about to write up an example using Getopt::Long, but found that toolic had already written an example using it a couple of years ago. With Getopt::Long, you have the ability to add optional arguments later, unambiguously.