Actually, both keys are strings and neither key is a compiled regexp. A strinfigied copy of what is in the curlies is used as the key.

use strict;
use warnings;

my %test;

my $re = qr/foo/;
my $s  = 'foo';

print('re: ', ref($re) || 'Not Regexp', "\n");
print('s:  ', ref($s ) || 'Not Regexp',  "\n");

$test{$re} = 1;
$test{$s } = 1;

foreach (keys %test) {
   print("key $_: ", ref($_) || 'Not Regexp', "\n");
}
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outputs

re:               Regexp
s:                Not Regexp
key (?-xism:foo): Not Regexp
key foo:          Not Regexp
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