Perhaps you meant the following? (I added {} around $usernumber) 
if( exists $xml->{$usernumber}->{"user"}->{"firstname"} ) {...}

[download]

EDIT: You may also need to extend it as such: (but it is difficult to tell for sure w/o seeing more context)

if( ref eq 'HASH' and exists $xml->{$usernumber}->{"user"}->{"firstnam
+e"} ) {...}

[download]

In reply to Re^3: Parsing hashes with variable structure by frozenwithjoy
in thread Parsing hashes with variable structure by SomeNoob

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