It acts like C's ternary operator.

If you don't know C, here's a little more explanation. :-) It evaluates a condition, and then returns one of two alternatives. You could think of it like an if(...){...}else{...} that looks funny. Some examples:

sub doIfTrue() { print "Doing true stuff...\n" }
sub doIfFalse() { print "Doing false stuff...\n" }

my $booleanVariable = false;

$booleanVariable ? doIfTrue() : doIfFalse();
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The if..else equivalent would be written as:

if ( $booleanVariable == true ) { doIfTrue() }
else                            { doIfFalse() }
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or:

if ( $booleanVariable ) { doIfTrue() }
else                    { doIfFalse() }
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That's one way to use it, but you can also do some cooler things with it (cooler in the sense that it's shorter, a little mind-bending, etc.):

...
my $daysRemaining = someFunction();
print "You have $daysRemaining ",
  $daysRemaining > 1 ? "days" : "day",
  " left to finish project X.\n";
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If $daysRemaining computes to 2, you will "You have 2 days left to finish project X." On the other hand, if $daysRemaining computes to 1, your output will be "You have 1 day left to finish project X."

I asked a related question once (while posing as Anynomous Monk, or maybe that was before I got an account...I don't rememember).

Update: Man, you guys are fast (or I am long winded)...in the time it took me to reply, there were three or four other responses alread in. :-)