XML::Simple already does a primitive pretty print in XMLout(). Here is how it's used with your data:

use XML::Simple;

my $doc = XMLin 'air.xml', forcearray => 1;
print XMLout $doc, xmldecl => 1, rootname => 'AIRAVAALABILITY';

__END__
prints:
<?xml version='1.0' standalone='yes'?>
<AIRAVAALABILITY>
  <AIRAVL>
    <AIRAVL007V>Y</AIRAVL007V>
    <AIRAVL018B>HKG</AIRAVL018B>
    <AIRAVL012U>01MAY</AIRAVL012U>
    <AIRAVL012G></AIRAVL012G>
    <AIRAVL0145>SIN</AIRAVL0145>
  </AIRAVL>
</AIRAVAALABILITY>
[download]

Update: fixed up root tag

After Compline,
Zaxo