To clear up a misconception though, your "action at a distance" horror scenario there won't happen. No sir. $a and $b are (apparently) local'd to the block:

@list=qw( one two three four five );

$a="Hello";
$b="World";
sort { $a <=> $b } @list;
print "$a $b";  # Hello World
[download]

Ah, but the "action at a distance" most certainly does occur. If you create a sorting sub in one module and use it in another ... Look at the following code:

my $a = 5;

my @b = reverse (0 .. 9);
print "@b\n";

sub boo {
$a <=> $b
}

@b = sort boo @b;
print "@b\n";
print $a,$/;
----
9 8 7 6 5 4 3 2 1 0
0 1 2 3 4 9 8 5 6 7
5
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Note the lack of sorting after 4. :-)

------
We are the carpenters and bricklayers of the Information Age.

Don't go borrowing trouble. For programmers, this means Worry only about what you need to implement.

While I don't have time to fully explain it, you can actually use prototypes(bleck) to avoid the weirdness in $a and $b (ughhh) for 5.6 and above....

sub boo ($$) {
  my ($a,$b) = @_;
  $a <=> $b
}
[download]

-Blake