push @{$wo_usr_proj{$project}}, $user;

##</code><code>##

foreach $proj(sort(keys(%wo_usr_proj)))
{
   print RHANDLE "$proj:\n";$wo_usr_proj{$proj}

   # note dereference: @{ $array_ref_here }
   foreach $element(sort @{ $wo_usr_proj{$proj} })
   {
      print RHANDLE "$element\n";
   }
   print RHANDLE "\n\n";
}

##</code><code>##

use Data::Dumper;
print Dumper \%wo_usr_proj;

__END__
$VAR1 = {
          'project2' => [
                          'WO',
                          'WO2
                        ],
          'project1' => [
                          'WO'
                        ]
        };

##</code><code>##

my $foo= "bar";
my $ref= \$foo;

print $$ref;

__END__
bar

##</code><code>##

my @array= qw(foo bar baz);
my $ref= \@array;

print join ':' => @$ref;

__END__
foo:bar:baz

##</code><code>##

my %h= ( foo => 'bar' );
my $ref= \%h;

while( my( $k, $v )= each %$ref ) {
  print "$k -> $v"
}

__END__
foo -> bar