(regarding $hashref->{-option})

Interestingly, the former is actually interpreted as an expression (note the "negate" below),...

Thanks for providing another interesting insight!

We've moved pretty far from the original issue, so I've modified the title... and I allow myself to add some stuff I found while experimenting with bareword stringification (and, just for fun, abusing some not directly related pitfalls into which I fell some time ago). Warning: This isn't to be taken too seriously. Read on at your own risk.

Consider the following code:

use strict;
use warnings;
use Data::Dump qw(dump);

$_ = $0;
my %h;

sub f { print('->f()') . '.0'; }

$h{  f  } =  '  F';
$h{ +f  } =  ' +F';
$h{+ f  } =  '+ F';
$h{ -f  } =  ' -F';
$h{- f  } =  '- F';
$h{  f()} =  ' ()';
$h{ -f()} =  '-()';
$h{- f()} =  '-()';
$h{ +f()} =  '+()';
print "\n",dump(\%h),"\n";
[download]

This prints:

->f()->f()->f()->f()->f()
{ "-1" => "-()", "-f" => "- F", "1" => "-()", "1.0" => "+()", "f" => "
+  F" }
[download]

The resulting hash %h has five keys, and the function f has been called five times. Before going into the boring stuff, see the following code, which is equivalent... well, almost:

use strict;
use warnings;
use Data::Dump qw(dump);

$_ = $0;
sub f { print('->f()') . '.0'; }

my %h = (
     f    => '  F',
    +f    => ' +F',
   + f    => '+ F',
    -f    => ' -F',
   - f    => '- F',
     f()  => ' ()',
    -f()  => '-()',
   - f()  => '-()',
    +f()  => '+()',
);
print "\n",dump(\%h),"\n";
[download]

...which prints...

->f()->f()->f()
{ "-1" => "-()", "-f" => "- F", "1" => "-()", "1.0" => "+()", "f" => "
++ F" }
[download]

This time I've used "fat comma" stringification which got me the same keys, but only three calls to function f.

So, these are the questions:

1. Why are there five keys in the hash, for eight apparently different expressions?
2. Which of the key definitions don't cause function f to be called in the second example?
3. Why is there a different value for $h{'f'}?
4. Why is there an odd number of calls to the function f in both examples?
5. Where do the keys -1, 1, and 1.0 come from?

I always wanted to test that Perlmonks' spoiler element, so here it is: