my $rec;
sysopen(DF, $ARGV[0], O_RDONLY | O_CREAT) or die "$ARGV[0] : $!";
sysread(DF, $rec, -s DF);
close DF;

open IN, '<', \$rec or die $!;
open OUT, '>', \my $out or die $!;
seek OUT, length( $rec )-1, 0;
print OUT ' ';
seek OUT, 0, 0;

while( <IN> ) {
    ## Do stuff to this line in $_; 
    m[ ];
    print OUT;
}

sysopen(DF,"test.txt", O_WRONLY | O_CREAT);
syswrite DF, $out;
close DF;