The two answers above should address the problem you're having. Here are some style tips though...

In these two lines, you are treating $self like it is a hashref...

$self->{tempfile} = File::Temp->new(UNLINK => 1, SUFFIX => '.tmp');
print $self->{tempfile} "I am here";
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But self is not a hashref; it's an object. OK, so Moose implements objects as blessed hashrefs by default, but it is considered bad form to treat an object as a hashref - we're supposed to pretend that it's not a hashref, and only access the internals via the accessors that Moose gives us. So we should do this...

$self->tempfile( File::Temp->new(UNLINK => 1, SUFFIX => '.tmp') );
print {$self->tempfile} "I am here";
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The reasons for doing so are not just theoretical. Accessing the object as a hashref bypasses all your type constraints, triggers, etc.

Secondly, you are initialising an attribute within the BUILD method. While you can do that, Moose does provide attribute defaults and builders for this sort of thing:

package test;   # not 'test.pm'
use Moose;
use File::Temp;

has tempfile => (
    is       => 'rw',
    isa      => 'File::Temp',
    lazy     => 1,
    default  => sub {
        File::Temp->new(UNLINK => 1, SUFFIX => '.tmp')
    },
);

sub BUILD {
    my $self = shift;
    print { $self->tempfile } "I am here";
}
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