An example of writing long JSON lines to a file atomically with Perl while warning about writes in excess of PIPE_BUF

safely_write.pl

use strict;
use warnings;
use JSON::XS ();
use POSIX qw(PIPE_BUF);

my $PATH = ...;
my $DATA = ...;
my $TIME = time;
my $JSON = JSON::XS->new->ascii(1)->indent(0);

open my $fh, ">>", $PATH or die "PANIC: Unable to open(<$PATH>): <$!>";
for my $item (@$DATA) {
    my $json = $JSON->encode($item);
    my $output = sprintf "%d\t%s\n", $TIME, $json;
    my $length = bytes::length($output);
    warn "PANIC: We shouldn't write items of size >PIPE_BUF. We're relying on atomic writes! Potentially writing corrupt data!"
        if $length > PIPE_BUF;
    syswrite $fh, $output or die "PANIC: Unable to syswrite() <$length> bytes of output: <$!>";
}
close $fh or die "PANIC: Unable to close(<$PATH>): <$!>";