One caveat:

package MyObj;

sub new {
   bless {}, shift;
}

sub test {
   print("test\n");
}


package main;

my $o = MyObj->new();

my %hash;
$hash{$o} = "This is my object";

foreach (keys(%hash)) {
   print("$_\n");
# MyObj=HASH(0x1abefc0)

   print("$hash{$_}\n");
# This is my object

   $_->test();
# Can't locate object method "test" via package
# "MyObj=HASH(0x1abefc0)" (perhaps you forgot to
# load "MyObj=HASH(0x1abefc0)"?) at script.pl line 26
}
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