I see that an dI think, look at the standard typemap , which has this

FILE *            T_STDIO
PerlIO *        T_INOUT
FileHandle        T_PTROBJ
InputStream        T_IN
InOutStream        T_INOUT
OutputStream        T_OUT
[download]

Then further down its

T_STDIO
    $var = PerlIO_findFILE(IoIFP(sv_2io($arg)))
T_IN
    $var = IoIFP(sv_2io($arg))
T_INOUT
    $var = IoIFP(sv_2io($arg))
T_OUT
    $var = IoOFP(sv_2io($arg))
[download]

So I'd look into IoOFP and sv_2io,

And with that I find http://perldoc.perl.org/perlxstypemap.html#T_INOUT and http://perldoc.perl.org/perlxstut.html#EXAMPLE-9-Passing-open-files-to-XSes

And then something something sleep :P