XMLout seems to expect a Perl structure not a string, so why don't you build it like this:

use strict;
use warnings;
use XML::Simple qw(XMLout);

my $VAR1 = "bla1";
my $VAR2 = "bla2";
my $VAR3 = "bla3";
my $VAR4 = "bla4";
my $xml = { 
            source=>[{
                    abc=>[{
                            uname=>[$VAR1],
                            dbcrdn=>[$VAR2],
                            paridn=>[$VAR3],
                            osath=>[$VAR4],
                        }] 
                    }]
            };
print XMLout($xml,RootName => "root");
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or more interactively

use strict;
use warnings;
use XML::Simple qw(XMLout);

my $VAR1 = "bla1";
my $VAR2 = "bla2";
my $VAR3 = "bla3";
my $VAR4 = "bla4";
my $abc;
push @$abc, { uname=>[$VAR1], dbcrdn=>[$VAR2], paridn=>[$VAR3], osath=
+>[$VAR4], };
my $xml;
push @{$xml->{"source"}}, { abc => $abc };
print XMLout($xml,RootName => "root");
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