Try this (works in some tests I did):

use strict;
use warnings;

sub report($@) {
  my $file = shift;
  my $temp = "$file.$$";
  1 until -w $file;
  rename $file, $temp or die "rename [$file] [$temp]: $!";
  open my $t, '+<', $temp or die "open [$temp]: $!";
  seek $t, 0, 2 or die "seek [$temp]: $!";
  print $t @_;
  close $t;
  rename $temp, $file or die "rename [$temp] [$file]: $!"
}
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As the two 'rename' operations are atomic, this might work, and the file inode is preserved, so all should be well. Change the "$file.$$" for other (better) tempfilename if you expect concurrent accesses by the same process or thru the network.