Less clutter and more readability, really. Kinda like Perl's trailing conditionals...
in reply to No braces
which then you can use to semi-duplicate in a twisted way C and Java's style:
$x = 8 if $x == 7;
Update Fixed code example as per Abigail-II's reply. Sorry about that, and yes, the first is less efficient, but we weren't talking about efficiency here, only style (not to say it isn't a good point to bring up).
$x = 0
unless $x == 7;
$x = 8
if $x == 7;
# ... or maybe ternary operator? ...
$x = $x == 7 ? 8 : 0;