Nice idea! But I don't like the idea that you have to pass in the name of the variable to be assigned, for its consequence that it can't be a lexical variable.

The idea is to 'require' a module and invoke some method of it in one call, right?
Here's my take:

# package 'Factory', exports function 'fab' (short for "fabricate")

package Factory;
use base 'Exporter';
@EXPORT = qw( fab );
use Carp qw(croak);
use strict;
use warnings;

sub fab($$@)
{
        my $class_name = shift;
        my $ctor_name = shift;
        $class_name =~ /[^\w:]/ and croak;
        eval 'require '.$class_name;
        $class_name->$ctor_name(@_)
}

1;
[download]

Use it in a one-liner:

perl -MFactory -le "my $x = fab 'My::Very::Long::Module', 'new', foo =
+> 42;  print $x->get_foo;"
[download]

Pedantry: Technically, Perl 5 (that is, its intrinsic object programming system) does not have constructors. What we typically name new are typically factory methods.
(I say "typically" because, by Wall, Perl gives you enough rope to shoot yourself in the foot.)