use strict;
use Fcntl qw( LOCK_EX SEEK_SET );

my $file = 'data.file';

open(my $FH, '>>', $file)
   or die("Can't open $file: $!\n");
flock($FH, LOCK_EX);
seek($FH, SEEK_SET, 0);
truncate($FH, 0);
...