our configthing $cfg = new configthing;
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You don't show us what your routine configthing::new looks like. This is where $cfg would get set. Maybe Data::Dumper can tell you more:

...
use Data::Dumper;
our $cfg = configthing->new();
print Dumper $cfg;
emonitor::list_hosts($cfg, \@hosts);
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I really, really wonder though, why you would need to (re)bless $cfg into configthing in the package emonitor.

package configthing;
use strict;
use Cwd 'getcwd';
use File::Basename;
use fields qw(
  # snipped for brevity...
);

our %DEFAULTS = (
  # snipped for brevity...
);

1;

sub new {

    my $this = shift;

    unless (ref $this) {
        $this = fields::new($this);
    }

    # properties (keys in the pseudohash) we set when an
    # object is initialized
    $this->{pname} = basename($0);
    $this->{pdir}  = dirname($0);
    if ( exists($ENV{'TERM'}) && '' ne $ENV{'TERM'} ) {
        $this->{is_interactive} = 1;
    }
    else {
        $this->{is_interactive} = 0;
    }

    # define empty values for all known configuration keys
    our %FIELDS;
    foreach my $k ( keys(%FIELDS) ) {
        $this->{$k} = '' unless defined($this->{$k});
    }

    # initialize default values
    our %DEFAULTS;
    foreach my $k ( keys(%DEFAULTS) ) {
        $this->{$k} = _expand_value($this, $DEFAULTS{$k});
    }

    return $this;
}
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doh! I'm mising the bless($this, 'configthing'); which explains why I put 33 separate bless($cfg) incantations all over the place. Dusty code that I obviously didn't understand when I wrote it.

I don't see anything immediately wrong in relation to the error at hand. Please do as JavaFan suggests and post a small program that reproduces the error.

You really, really should look over this code still, because it (re)declares our %DEFAULTS in two places which makes very little sense as %DEFAULTS is still visible from above.

The first thing you should do is show what print Dumper $cfg; says inside your list_hosts function.

If in doubt, print it out!

This seems to be working correctly, and properly coded now. I've gone through and...

- Removed all the extraneous bless($cfg,'configthing') statements (eg, the line 190 in the original error message) that were scattered all over the original code.

- Added require 5.10.0; statements to the various main files to prevent inadvertently going back to 5.8.8 (which is the OS's perl installation.)

As Corion pointed out, this thing needs a review . . .

Trimming down, even before I have a small case that shows the error I have something else wrong. Here're two files that give me different results on 5.8.8 versus 5.10. I think I've already done something wrong at this level of simplicity that I should figure out before building the example up farther.

use strict;
use warnings;

use configthing qw();

our configthing $cfg = new configthing;

use Data::Dumper;
print Dumper $cfg;
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Then for "configthing.pm"...

package configthing;

use strict;
use warnings;

use fields qw(
  a
  b
  c
  d
);

our %DEFAULTS = (
    b => 'default for b',
    c => 'default for c'
);

1;

sub new {
    my $this = shift;

    unless (ref $this) {
        $this = fields::new($this);
    }

    our %FIELDS;
    foreach my $k ( keys(%FIELDS) ) {
        $this->{$k} = '' unless defined($this->{$k});
    }

    $this->{a} = 'set at initialization';

    foreach my $k ( keys(%DEFAULTS) ) {
        $this->{$k} = $DEFAULTS{$k};
    }

    return $this;
}
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With 5.8.8 I get

$VAR1 = bless( [
                 bless( {
                          'c' => 3,
                          'a' => 1,
                          'b' => 2,
                          'd' => 4
                        }, 'pseudohash' ),
                 'set at initialization',
                 'default for b',
                 'default for c',
                 ''
               ], 'configthing' );
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And 5.10.1 gives me:

$VAR1 = bless( {
                 'c' => 'default for c',
                 'a' => 'set at initialization',
                 'b' => 'default for b',
                 'd' => ''
               }, 'configthing' );
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This is completely to be expected in a change from 5.8 to 5.10 because pseudohashes do not exist anymore in 5.10. The implementation of fields was changed to shield you from this other change I believe.