Doesn't seem to create a sparse file:

[tmp]$ cat broomduster.pl
open my $fh, '>', 'zeros'  or die "error opening fh: ($!)";
binmode $fh;
print $fh pack "x".(1024*1024*10);

[tmp]$ rm zeros; perl broomduster.pl; ls -lh zeros; du -h zeros
-rw-rw-r-- 1 duelafn 10M 2008-10-09 22:17 zeros
11M     zeros
[download]

However, the following works:

[tmp]$ cat duelafn.pl
open my $fh, '>', 'zeros'  or die "error opening fh: ($!)";
seek $fh, 1024*1024*10 - 1, 0;
print $fh "\0";

[tmp]$ rm zeros; perl duelafn.pl; ls -lh zeros; du -h zeros
-rw-rw-r-- 1 duelafn 10M 2008-10-09 22:17 zeros
12K     zeros
[download]

This would not create a 100% sparse file because the last block would be all zero (even you print only one zero). Fletch was right in his above post, the combination of seek and truncate must be used.

Here my first attempt from yesterday evening to create a halfway secure sub function which doesn't mess-up existing files:

sub create_sparse {
    my ($file, $size) = @_;
    return if -e $file && ! -f _;
    return -2 if -e _ && -s _ >= $size;

    open my $fh, '+>', $file or return;
    eval {
        seek $fh, $size, 0 or die;
        truncate $fh, $size or die;
    } or do {
        close $fh;
        return;
    };
    close $fh or return;
    return -1;
}
[download]

Just curious:

• Why the need for seek when you're just going to close $fh after truncate?
• Please expain the eval - close $fh - return logic you're using.