Perhaps param() is using a bare return on error. A bare return returns undef or the empty list, depending on the context in which the subroutine was called! Your second example calls param() in scalar context, so a bare return would give you undef; Your first example calls param() in list context, so a bare return would give you (). To help confirm my diagnosis, try replacing your anon-hash constructors {...} with anon-array constructors [...], and impose scalar context by using scalar directly. By avoiding the hash, you will prevent confusion from the random hash ordering in your dump.

use strict;
use warnings;
...
my $dump_string = Dumper [
### Uncomment one or the other of these two lines.    
#   edit_usr_id =>         $self->client->param( 'edit_usr_id' ),
    edit_usr_id => scalar( $self->client->param( 'edit_usr_id' ) ),

    mode        =>         $self->client->param( 'mode' )
];
$self->log(LOGDEBUG, 'EDIT_USER: '.$dump_string ));
[download]

It should give something like:

 EDIT_USER: $VAR1 = [
    'edit_usr_id',
    undef,
    'mode',
    'add',
  ];
[download]

or:

EDIT_USER: $VAR1 = [
   'edit_usr_id',
   'mode',
   'add',
 ];
[download]

Also, you should have gotten a warning about 'Odd number of elements' in your hash assignment in your first example. You do have warnings turned on, don't you?