while ( $indata =~ s{<file fiop="([^"]+)" length="(\d+)"/>(.*?)</file>}{}s )
{
    ( $fiop, $fileLength, $fileData ) = ( $1, $2, $3 );
    # do something with $fileData, possibly after checking that
    # $fileLength == length( $fileData ), if that matters to you
}
if ( $indata =~ m{<file fiop=|</file>} )
{
    # there's something wrong with the posted data, so it's still
    # not suitable for XML parsing...
}