Update: Small changes applied to the two examples.

Having each worker obtain the file handle will do the trick. The + is not necessary in this context but needed for LOCK_SH. Thus, I typically open flock handles in read and write mode.

open ($fh, '+>>', $file_out);

flock($fh, LOCK_EX) or die "cannnot lock file - $!\n";
            print $fh @out;
            #print $fh $out[0]; #Date
            #print $fh $out[1]; #Site
            #print $fh $out[2]; #CTR
            #print $fh $out[3]; #IP 
            #print $fh $out[4]; #Temperature
flock($fh, LOCK_UN) or die "cannnot unlock file - $!\n";

close $fh;
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Below is an example using MCE::Loop. Calling MCE->print passes data to the manager process to append to the file. Locking is handled automatically behind the scene.

use Net::SNMP;
use MCE::Loop;

# ... acquire devices and log handle

open ($fh, '>>', $file_out);
print $fh "$date;Site;CTR;IP;Temperature\n";

MCE::Loop::init( chunk_size => 1, max_workers => 50 );

mce_loop {
    my $key_bgtr = $_;
    my @out;

    foreach my $key_ip (keys %{$devices{$key_bgtr}})
        {
        $out[0]=$date.';';
        $out[1]=$devices{$key_bgtr}{$key_ip}.';';
        $out[2]=$key_bgtr.';';
        $out[3]=$key_ip.';';

        for (my $k =2;$k<=3;$k++)
            {
            if (!defined $out[$k]) {$out[$k] = './.'}
            }

        $ss =  Net::SNMP->session(Hostname=> $key_ip, Timeout => 1);

        $out[4] = $ss->get_request(Varbindlist => [$oid]);
        $out[4]= $out[4]->{$oid};

        if (defined $out[4] && $out[4] !~ /^$|\s/ && $out[4] >=5)
            {
            $out[4] = "$out[4]\n";
            }
        else
            {
            $error = $ss->error();
            if ($error =~ /.*noSuchName.*/)
                {
                $out[4]="no ALCplus2e\n";
                }
            else
                {
                $out[4]="OFFLINE\n";
                }
            MCE->print($log, "$key_bgtr: $error\n");
            }

        MCE->print($fh, @out);
        $ss->close;
        }

    } keys %devices;

close $fh;
[download]

The following example outputs orderly if processing sorted keys is desired. Each worker must gather regardless if @out is empty or not. The manager process needs the $chunk_id value for ordered output.

use Net::SNMP;
use MCE::Loop;
use MCE::Candy;

# ... acquire devices

open ($fh, '>>', $file_out);
print $fh "$date;Site;CTR;IP;Temperature\n";

MCE::Loop::init(
    chunk_size => 1, max_workers => 50,
    gather => MCE::Candy::out_iter_fh($fh)
    );

mce_loop {
    my ($mce, $chunk_ref, $chunk_id) = @_;
    my $key_bgtr = $chunk_ref->[0];
    my @out;

    foreach my $key_ip (keys %{$devices{$key_bgtr}})
        {
        $out[0]=$date.';';
        $out[1]=$devices{$key_bgtr}{$key_ip}.';';
        $out[2]=$key_bgtr.';';
        $out[3]=$key_ip.';';

        for (my $k =2;$k<=3;$k++)
            {
            if (!defined $out[$k]) {$out[$k] = './.'}
            }

        $ss =  Net::SNMP->session(Hostname=> $key_ip, Timeout => 1);

        $out[4] = $ss->get_request(Varbindlist => [$oid]);
        $out[4]= $out[4]->{$oid};

        if (defined $out[4] && $out[4] !~ /^$|\s/ && $out[4] >=5)
            {
            $out[4] = "$out[4]\n";
            }
        else
            {
            $error = $ss->error();
            if ($error =~ /.*noSuchName.*/)
                {
                $out[4]="no ALCplus2e\n";
                }
            else
                {
                $out[4]="OFFLINE\n";
                }
            MCE->print($log, "$key_bgtr: $error\n");
            }

        $ss->close;
        }

        MCE->gather($chunk_id, join('', @out));

    } sort keys %devices;

close $fh;
[download]

Kind regards, Mario