Hi

Thanks for your comments. The error is due to an empty line at the end of the __DATA__section. I must have added it when I pasted the code.

Since the empty line did not match /\d:\d:\d$/, the data for that host was considered to be worth keeping.

I trust the issue is now sorted. I Simply added

next if $a_line =~ /^\s*$/; #ignore empty lines
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after reading each line.

Here is the full code, with the extra line check, and the empty __DATA__ line.

use strict;
use warnings;
use 5.010;

my @tmp;
my @keep;

while (my $a_line = <DATA>) {
    next if $a_line =~ /^\s*$/; #ignore empty lines
    chomp $a_line;
    if ( $a_line =~ /^\d+\s+\D+/ ) 
    {
            #we have a new host master record
            #process the data from the previous host
        process_previous(\@tmp);
            #on return @tmp may be empty

        @keep = (@keep, @tmp);
        @tmp =(); #now we empty it anyway
    }
    push @tmp, $a_line;
}

process_previous(\@tmp);

@keep = (@keep, @tmp);
@tmp=();

print "$_\n" for @keep;
exit(0);

#----------------- SUBS ----------------------------
sub process_previous {
    my $array_ref = shift;
    my $keep_this_data = 0;
    foreach my $elem(@$array_ref) 
    {
        if ($elem !~ /\d:\d:\d$/ ) 
        {
            #found a line NOT terminating in 3 digit, each separated b
+y a
            #colon, so we want to keep the whole info abou this host
            $keep_this_data = 1;
            last;
        }
    }

    if (!$keep_this_data) 
    {
        #empty the array
        @$array_ref = ();
    }
}

__DATA__
7 hostname12 Generic-legacy 10000000AB210ACF6 ---
                             10000000AB210ACF4 2:5:4
9 hostname13 Generic        10000000AB2A3006A 3:5:2
                             10000000AB2A30068 2:5:2
23 srvernam   Generic-legacy 5001438002A3004A 3:3:3
                             5001438002A3004A ---
                             5001438002A30048 2:3:3
                             5001438002A30048 2:5:2
                             5001438002A30048 2:5:2
9 hostname13 Generic        10000000AB2A3006A 3:5:2
                             10000000AB2A30068 2:5:2
21 HOSTNAME   Generic
9 hostname13 Generic        10000000AB2A3006A 3:5:2
                             10000000AB2A30068 2:5:2

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Kind regards