Try this version of your code that uses Data::Dump and things become obvious:

#! perl -slw
use strict;
use warnings;
use Storable qw(dclone);
use Data::Dump qw[ pp ];

my $str = "Rico";

$str =~ m{(?<name>ico)}xms;

my $match = dclone \%+;

print \%+,;
print " match:  $match -",  pp $match;

$str =~ m{(?<buba>R)}xms;

my $match2 = dclone \%+;

print \%+;

print " match:  $match ",  pp $match;
print "match2: $match2 ",  pp $match2;

__END__
C:\test>816700
HASH(0x22ee20)
 match:  HASH(0x6f218) -{
  # tied Tie::Hash::NamedCapture
}
HASH(0x22ee20)
 match:  HASH(0x6f218) {
  # tied Tie::Hash::NamedCapture
}
match2: HASH(0x6f128) {
  # tied Tie::Hash::NamedCapture
}
[download]

%+ is not a real hash, but rather a tied hash. And cloning it appears to simply gives you another tied reference to the same global internal data structure.