Having put the code in a small test program there's nothing obviously 'bizarre' about it running under v5.6.1

package rincewind;

sub new { bless {}, shift }

sub jil {
    my ($self) = @_;

    my $outstr = $self->_jilout('insert_job') .
            $self->_jilout('job_type');

    $outstr .= $_
      foreach
        map $self->_jilout($_), @$self{ @{$self->{_fields_in_seq}} };

    $outstr;
}

sub _jilout {
    my ($self,$attr) = @_;

    return '' if !exists $self->{$attr} or !defined $self->{$attr};
    "$attr: ".$self->{$attr}."\n";
}

package main;

use strict;
use warnings;
use Data::Dumper;

my $o = rincewind->new;

@$o{qw/insert_job job_type/} = qw[ 50 daemon ];

my @flds = qw[ foo bar baz quux ];

@$o{ @flds } = reverse @flds;
@{ $o->{_fields_in_seq} } = @flds;

print $o->jil, $/;

exit 0;

__output__

insert_job: 50
job_type: daemon
quux: foo
baz: bar
bar: baz
foo: quux
[download]

I can't see any attempts to copy an internal variable that isn't copyable^[1]. Maybe some version info would help?
HTH

_________
broquaint

^[1] paraphrased from perldiag