Perl 6:

my $x = "this is a test";
$x ~~ s:g/\s/-/;
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Notes:

• P6 uses strict mode by default, so I've added a 'my'.
• The P6 smartmatch operator (~~) covers many cases including ones covered by =~ in Perl 5.
• In P6 :g is a regex modifier, in this case making the search/replace global. (In Perl 5 it would have been specified as s///g.)
• In effect P6 regex all have the equivalent of Perl 5's /x regex modifier set on. You can't switch it off. The \s is one of several whitespace alternatives available.

If you want any further help regarding Perl 6, I strongly recommend a visit to the IRC channel #perl6 on freenode, but I'all also check in here to see if you post a follow up.

Hth.