does it work with things besides return?

return unless ($i <= 0);
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print "i = $i\n" while ($i++ < 10);
$i = 0;
print "i = $i\n" until ($i++ >= 10);
print "$_\n" for (1..10); # note: for my $i ... doesn't work
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It also works with trinaries as well.

return $i == 1 ? 'red'
     : $i == 2 ? 'blue'
     : $i == 3 ? 'green'
     : $i == 4 ? 'yellow'
     :           'white';
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However, you should only use this sort of syntax if the syntax is short. It easier for a person to read columns up and down then it is from something that goes from one end of the page to the other. Lets change my example to so you see what I mean.

return $i == 1 ? 'red': $i == 2 ? 'blue' : $i == 3 ? 'green'     : $i 
+== 4 ? 'yellow' : 'white';

# God forbid
print "This is my very long string that I wish to print out to display
+ my message on the screen but only when I want it too!" if $i == 1;
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The last one is just asking for trouble because you will probably be looking at the print on the front and not notice the if statement on the end.

Well, for equality testing, I'd use a hash (or array for dense numerical indexes) lookup.

my @COLORS = qw( white red blue green yellow );
return exists $COLORS[$i] ? $COLORS[$i] : $COLORS[0];
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But if you need logical checks that are more complex than equality, the ternary chain you show is quite handy:

return $i < 5    ? 'red'
     : $i < 20   ? 'blue'
     : $i%3 == 0 ? 'green'
     : $i%5 == 0 ? 'yellow'
     :             'white';
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And you are absolutely correct about the importance of formatting with this construct.

As for the trailing 'if','unless' and 'or', formatting is key here as well. I tend to bring those onto a separate line:

print "This is my very long string that I wish to print out to display
+ my message on the screen but only when I want it too!" 
    if $i == 1;

# the very common 'or die' idiom with a big brutish open()
open ( 
    my $filehandlewithalongname,
    $modevariablewithalongname, 
    $pathvariablewithalongname
) or die "Unable to open file $pathvariablewithalongname: $!\n";

# 'or die' again with a smaller, kinder open:
open ( my $fh, $mode, $path )
  or die "Unable to open $path: $!\n";

# is equivalent to this 'die unless'
# which I have never seen used with open().
die "Unable to open $path: $!\n" 
    unless open ( my $fh, $mode, $path )

# however, 'die unless' is common with value checks.
die "Illegal value: $rotifer\n"
  unless $rotifer =~ /^is ok$/;

# as is 'die if'
die "Illegal value: $rotifer\n"
  if $rotifer =~ /^is not ok$/;
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I think the main difference between 'x or y' and the equivalent 'y unless x' is emphasis. In the former, the emphasis is on the statement 'x'. Whereas in the latter, the emphasis is on the statement 'y'. Choice of form is therefore indicated by what you believe will express your intent most clearly.

Another thing worth noting here is that one can use 'x and y' as a stand-in for 'y if x'. I avoid 'x and y' because the short circuit evaluation of 'and' is, in my opinion, not intuitive. If I want to emphasize the 'x' in the code, I just use the good old 'if(x) { y }' form instead of 'x and y'.


TGI says moo

Using this "backwards" style, your code intent stays more visible.

For example, it's common to do this for something like:

print "Here we are..." if $debugging;
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if ($unlikely_condition) { &do_that_voodoo; }
&that_yoodoo if $typical_case;
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Personally I use postfix only in case the whole block fits in one line.

Yeah, classic 2005

As others have pointed out this is Perl's "statement modifier" idiom. Perl has many shorthand ways of achieving things and that is a large part of what gives Perl its power to be concise and clear.

consider the following:

my $filename = "$root/$path/$name";
open my $fileHandle, '<', $filename or die "Failed to open $filename: 
+$!";
print $fileHandle "$_:    $results[$_]\n" for 1 .. $#results;
print $fileHandle "Total: $results[0]\n";
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which deflares a variable in the middle of a file open statement, uses the "or die" idiom to check the result of an open, uses a for loop taking a range as a statement modifier and uses string interpolation to construct a file name from components and to build the strings to be printed.

The result is concise, lucid and robust.