This got me curious

$ perl -MO=Deparse -e 'print $q -> start_html(-title => "The Time", -b
+gcolor => "#ffffff") '
print $q->start_html(-'title', 'The Time', -'bgcolor', '#ffffff');
-e syntax OK

$ perl -MO=Deparse -e 'print -"asdf"'
print '-asdf';
-e syntax OK

$ perl -MO=Deparse -e 'print -"1asdf"'
print -1;
-e syntax OK

$ perl -MO=Deparse -e 'print -"-1asdf"'
print '+1asdf';

$ perl -MO=Deparse -e 'print -"+1asdf"'
print '-1asdf';
-e syntax OK
[download]

What I glean from this is that if you use unary minus on any non-numish-looking string constant, the minus will get prepended. If you use an undecorated(+/-) numish string constant it gets numified, but if you use a decorated one it inverts the sign and doesn't numify?

Is this accurate?
Is this documented anywhere?
What is the rule on this?
And what is this useful for (beyond use given in the OP)

Did you read Symbolic-Unary-Operators?

$ perl -MO=Deparse -e 'print -"asdf"'
print '-asdf';
-e syntax OK
[download]

If the operand is an identifier, a string consisting of a minus sign concatenated with the identifier is returned.

$ perl -MO=Deparse -e 'print -"1asdf"'
print -1;
-e syntax OK
[download]

If, however, the string begins with a non-alphabetic character (excluding "+" or "-"), Perl will attempt to convert the string to a numeric and the arithmetic negation is performed.

$ perl -MO=Deparse -e 'print -"-1asdf"'
print '+1asdf';

$ perl -MO=Deparse -e 'print -"+1asdf"'
print '-1asdf';
-e syntax OK
[download]

Otherwise, if the string starts with a plus or minus, a string starting with the opposite sign is returned.

--
seek $her, $from, $everywhere if exists $true{love};

Symbolic_Unary_Operators

Symbolic_Unary_Operators

You sure the - stringifies? Try this:

use strict;
use warnings; 
use Data::Dumper;

my %a = (-keys   => 'v1',
         -values => 'v2');
         
print Dumper(\%a);
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Gives (as expected):

$VAR1 = {
          '-keys' => 'v1',
          '-values' => 'v2'
        };
[download]

keys and values are (of course) builtins. When I replace the 'fat comma' with an ordinary one:

my %a = (-keys   , 'v1',
         -values , 'v2');
[download]

I get:

Not enough arguments for keys at C:\gash.pl line 6, near "keys   ,"
Not enough arguments for values at C:\gash.pl line 7, near "values ,"
[download]

It was the fat comma doing the stringification, not the -.

Update: Re-reading the previous post, Anon did say "things that look like strings", which is correct. keys and values "look like" Perl builtins. Changing the names gives different results:

my %a = (-xkeys   , 'v1',
         -xvalues , 'v2');
[download]

Gives:

$VAR1 = {
          '-xvalues' => 'v2',
          '-xkeys' => 'v1'
        };
[download]

Which I guess means it is not a good idea to rely on - to do stringification.

It, A fat comma string-ifies what's on the left of it...that only if it looks like a string.