This would work as intended:

use strict;
use warnings;

package Foo;

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

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

package main;

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

print "Done.\n";
[download]

# Created Foo=CODE(0x82cafec)
# Hello!
# DESTROYing Foo=CODE(0x82cafec)
# Done.
[download]

Or use closures:

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