return $is_true ? 'is true!' : 'is false';

# is just a one line way of saying
if ($is_true) {
  return 'is true!';
}
else {
  return 'is false!';
}
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Let me elaborate a bit more (aka update):
You can even nest them:

$a = $is_true ? 'is true!' : $was_true ? 'was true' : 'was never true'
+;
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The 'hook' operator (as i like to call it) is best used for assignments, as my examples showed. Don't be tempted to do this, as it is just bad coding practice:

$some_boolean ? do_sub1() : do_sub2();
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--------------------------------------------------

perl -le '$x="jeff";$x++ for(0..4482550);print $x'

At the risk of beating a dead horse:

The ternary expression is an expression, whereas the if equivalent is a statement.

In even more other words, the if statement cannot be used as a component of an expression while the ternary operator can.

So, this:

my $x = $is_true ? 'is true!' : 'is false';

may be considered a shorter way of saying this (note that the my $x; must appear outside of the if):

my $x;
if ($is_true) {
  $x = 'is true!';
}
else {
  $x = 'is false!';
}
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But, one cannot always simply condense an arbitrary if statement into a ternary expression

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In simple terms, it works like an if statement. The ternary operator has three parts (hence the name!) that look like this:

condition ? expression1 : expression2
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The condition is evaluated. If it it true then expression1 is returned, otherwise expression2 is returned. Here's a simple example:

$max = $x > $y ? $x : $y;
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The condition $x > $y is evaluated. If this is true then the operator returns $x, otherwise it returns $y. This could also be done using if like this:

if ($x > $y) {
  $max = $x;
} else {
  $max = $y;
}
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But in this case I think that the ternary operator looks more readable.

Perl Training in the UK <http://www.iterative-software.com>

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. :-)

Don't worry t'mo! Your answer was very complete and well worth the extra effort. As you point out, the ternary operator is very useful anywhere you might put an expression. Which means you can do a 'my' with a ternary assignment, which means you can put it in a print statement's argument list, which means you can do return wantarray ? @list, $list[0]; at the end of your subs to check context and return appropriately... and all sorts of other useful stuff where an if-then is not only cumbersome, but completely unexpressive.

$a = 1 ? 'abc' : 'def';  # sets $a to 'abc'
$a = 0 ? 'abc' : 'def';  # sets $a to 'def'
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Actually, if the second and third arguments are valid "lvalues" (things what you can assign stuff to), then you can have the ternary expression on the left hand side of an assignment operation. For example, consider the following:

($cheque_account_sel ? $cheque_balance : $card_balance ) -= $payment;
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Why would you want to use that? I don't know, but it is there.

Suitably described above. There is one handy case not mentioned as I begin to type this

$bobscats=2;
$billscats=1;
printf "Bob has %s\n", catstat($bobscats);
printf "Bill has %s\n", catstat($billscats);

sub catstat
{
  my($count)=@_;
  sprintf "%d %s", $count, $count>1?'cats':'cat';
}
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will print out Bob has 2 cats
Bill has 1 cat
So that it doesn't appear that you aren't sure how to say things like xxx has 1 cat(s)

In C mode, that's what I usually do, only slightly different:

printf ("%d %s%s\n", $count, "cat", ($count==1)?"":"s");
printf ("%d %s%s\n", $count, "cact", ($count==1)?"us":"i");
printf ("%d %s%s\n", $count, "Elvi", ($count==1)?"s":"i");
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As an alternative, you might want to use:

print $count, "cat", (($count==1)?"":"s"), "\n";
print $count, "cact", (($count==1)?"us":"i"), "\n";
print $count, "Elvi", (($count==1)?"s":"i"), "\n";
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Or perhaps as a method:

sub quantize
{
     my ($quantity, $singular, $plural) = @_;
     return (1==$quantity)? $singular : $plural;
}

$count = 2;

print "$count @{[quantize($count,'cat','cats')]}\n";
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