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

my %a = (-keys   , 'v1',
         -values , 'v2');
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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 ,"
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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');
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Gives:

$VAR1 = {
          '-xvalues' => 'v2',
          '-xkeys' => 'v1'
        };
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Which I guess means it is not a good idea to rely on - to do stringification.