For function calls (i.e. non-OO) the parentheses are sometimes necessary. I can never remember the exact formulations - depends very much on whether strict subs are enabled, and on prototypes. For function calls, best to always include the parentheses, unless it's a constant sub, or you are sure that the sub has been defined with an empty prototype.     [emphases added]

Unfortunately, there is a happy little cluster of corner cases associated with the  => 'fat comma' operator (see perlop) that are not affected by (or perhaps I should say, that interact in a complex way with) prototyping or strictures.

In the example below,  FOO() and  'FOO' (quoted string) are never surprising, but try commenting out successive hash elements from right to left. Try replacing  +FOO with  !FOO or  -FOO instead. Replacing the
    use constant FOO => 'foo';
statement with the prototyped function
    sub FOO () { 'foo' }
(which is essentially what constant creates for you) makes no difference.

Happy debugging!

>perl -wMstrict -le
"use constant FOO => 'foo';
 ;;
 my %hash = (
   FOO() => 'ok',  FOO => 'oops',  +FOO => 'huh?',  'FOO' => 'yes',
   );
 ;;
 use Data::Dump;
 dd \%hash;
"
{ foo => "ok", FOO => "yes" }
[download]

Yes, as you can see from this demonstration code:

#!/usr/bin/perl -w
use strict;

my $list = '
    FUD() => "()", "FUD" => "str", FUD => "bare", -FUD => "-", +FUD =>
+ "+",
';

sub FUD { 'fud' }
for(  0 .. 1  ) {
    my @a = eval $list;
    print "@a\n";
    last if $_;
    undef &FUD;
    eval 'sub FUD() { "dud" }';
}
__END__
fud () FUD str FUD bare -FUD - FUD +
dud () FUD str FUD bare -FUD - FUD +
[download]

While the FUD() case always calls the function (and thus gives 'fud' then 'dud') and the "FUD" case always doesn't call the function (and thus always gives 'FUD'), the other bareword cases are affected by whether or not a prototype is used and so give the follow inconsistent results:

                  FUD      +FUD     -FUD
                 -----     -----   ------
w/ prototype:     FUD       FUD     -FUD
w/o prototype:    FUD       FUD     -FUD
[download]

Happy debugging!

Like anybody could possibly debug such wily inconsistency! So funny.

Update: Sorry, I accidentally reversed the "w/" vs "w/o" labels in the above table. Sorry for the confusion that must have caused.

- tye        

... the other bareword cases are affected by whether or not a prototype is used ...

I don't get this point. While certainly inconsistent, the inconsistencies do not seem to be affected at all by whether prototyping is or is not used.

thanks for your detailed reply. I have one more question. What do the syntax +FUD and -FUD mean here?

You may be interested in the "winking fat comma"...

use constant KEY => 'foobar';
use Data::Dumper;
print Dumper({ KEY ,=> 'value' });
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Looks like a fat comma, works like a regular comma.

perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'

And then there's also the one they call the "weeping programmer":

>perl -wMstrict -le
"use constant KEY => 'foobar';
 use Data::Dumper;
 print Dumper({ KEY ,,,,,, 'value' });
"
$VAR1 = {
          'foobar' => 'value'
        };
[download]

I'm reminded of eval'ing coderef with ampersand, where I learned:

sub Q { print "Q" };
eval \&Q;   # <nothing>
eval \Q;    # "Q"
eval \Q();  # "Q"
eval \&Q(); # "Q"
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