Errr ... no, it is not:

perl -we 'bless( \sub {}, "Foo" )->();'
Not a CODE reference at -e line 1.
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perl -we 'print bless( \sub {}, "Foo" ) . "\n";'
Foo=REF(0x82998c4)
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Update: To answer your question: What seems to be happening is that perl inlines the subroutine. See:

use strict;
use warnings;

package Foo;

sub new {
    my $class = shift;
    return bless( sub { print "Hello\n" }, $class );
}

sub DESTROY {
    print "DESTROYing $_[0]\n";
    return;
}


package main;

my $object = Foo->new;
print "Created $object\n";

undef $object;
print "Ha! Ha! Still there!\n";

{
    no strict 'refs';
    my $symbol_table = \%{'Foo::'};
    delete $symbol_table->{'new'};
}

print "Done.\n";
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# Created Foo=CODE(0x82b682c)
# Ha! Ha! Still there!
# DESTROYing Foo=CODE(0x82b682c)
# Done.
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