use File::ReadBackwards;

my $listname = shift;

## Not sure whether F::RB allows you to specify an open mode?
tie *LIST, 'File::ReadBackwards', $listname or die $!;

while( <LIST> ) {
    my( $datafile, $position ) = split ' ';
    my $writePos = tell LIST; 
    truncate LIST, writePos;
    open DATA, '<', $datafile or die $!;
    seek DATA, $position, 0 if defined $position;
    while( <DATA> ) {
        seek LIST, $writePos, 0;
        print LIST ' ', tell( DATA ), "\n";
        
        ## Do stuff with the line from DATA in $_
    }
    close DATA;
    seek LIST, $writepos, 0;
    truncate LIST, $writePos;
}
close LIST; ## Should be empty by the time we get here.
unlink $listname;