If you recognise that the body of the while loop is effectively a subroutine with implicit parameters, then it becomes natural to think of what you describe as (crudely):

doBody( $_ ) while( <FILEHANDLE> );
doBody( undef );
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Which makes it clear that

1. you are calling (some part(s) of) the body from two different locations;
2. you want the body to take different actions dependent up when it is called;

   Ie. When processing the file, or afterwards.

3. Your "$_ == NULL" is a flag used just to decide when to do which type of processing.

That probably indicates that there is some subset of the processing inside the body of the loop that should really be factored into a separate subroutine, rather than inlined. Leading to

while( <FILEHANDLE> ) {
    do some stuff;
    doTheFactoredStuff();
    do some other stuff;
}

doTheFactoredStuff();
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Which may suggest a better way of doing things--or not:)