$ perl -Mstrict -Mwarnings -e '
> sub truncZero
> {
>     my $fh = shift;
>     truncate $fh, 0 or die $!;
>     seek $fh, 0, 0 or die $!;
> }
> 
> open my $fh, q{>}, q{rubbish} or die $!;
> 
> print $fh qq{Mary had a little lamb\n};
> print qx{/usr/bin/hexdump -vC rubbish}, qq{-----\n};
> 
> truncZero( $fh );
> print qx{/usr/bin/hexdump -vC rubbish}, qq{-----\n};
> 
> print $fh qq{The boy stood on the burning deck\n};
> print qx{/usr/bin/hexdump -vC rubbish}, qq{-----\n};
> 
> truncZero( $fh );
> print qx{/usr/bin/hexdump -vC rubbish}, qq{-----\n};
> 
> print $fh qq{I met a traveller from an antique land\n};
> print qx{/usr/bin/hexdump -vC rubbish}, qq{-----\n};
> 
> close $fh or die $!;'
00000000  4d 61 72 79 20 68 61 64  20 61 20 6c 69 74 74 6c  |Mary had a littl|
00000010  65 20 6c 61 6d 62 0a                              |e lamb.|
00000017
-----
-----
00000000  54 68 65 20 62 6f 79 20  73 74 6f 6f 64 20 6f 6e  |The boy stood on|
00000010  20 74 68 65 20 62 75 72  6e 69 6e 67 20 64 65 63  | the burning dec|
00000020  6b 0a                                             |k.|
00000022
-----
-----
00000000  49 20 6d 65 74 20 61 20  74 72 61 76 65 6c 6c 65  |I met a travelle|
00000010  72 20 66 72 6f 6d 20 61  6e 20 61 6e 74 69 71 75  |r from an antiqu|
00000020  65 20 6c 61 6e 64 0a                              |e land.|
00000027
-----
$ cat rubbish 
I met a traveller from an antique land
$