I use !!$x for this. Perl6 will give us ?$x, which ought to be clearer.

If I wanted to store the boolean back into the same variable, I might prefer a mutator: $x &&= 1.

However, there is a real difference:

sidhekin@blackbox:~$ perl
my ($x, $y) = (0, 0);

$x = $x && 1; # or $x &&= 1;
$y = !!$y;

print "\$x: '$x'\n\$y: '$y'\n";

__END__
$x: '0'
$y: ''
sidhekin@blackbox:~$
[download]

... something to be aware of :-)