My hint is to not trust your logging further up and to move the logging to where the problem is. If $data is undef, then $datapair does not contain a =.

So, investigate $datapair and the contents of @data just before your loop.

Maybe/most likely, $line is not what you think it is. Maybe it ends with a comma and a newline, and split helpfully keeps that newline as its own element in @data?

Thanks for the input! I just ran it again. Here is what $line (one example) looks like in my debug log (obscured for privacy):

"line: label1=N,label2=N,label3=N,label4=NNNNNNNNNNN,label5=N,label6=W
+WWN"
[download]

So it looks as though $line is populating properly... hmmmm....

Then please show us the relevant code+data that produces the output.

I can't reproduce the problem using the following, very short program:

#!perl
use strict;
use warnings;

my $line = "line: label1=N,label2=N,label3=N,label4=NNNNNNNNNNN,label5
+=N,label6=WWWN";

my @data = split(',', $line);
foreach my $datapair (@data) {
   my ($label, $data) = split('=', $datapair);
   print "label: $label; data: $data\n";
   #print OUT1 "$data;";
}

__END__
label: line: label1; data: N
label: label2; data: N
label: label3; data: N
label: label4; data: NNNNNNNNNNN
label: label5; data: N
label: label6; data: WWWN
[download]

... to me, that means, you are either doing something differently in your code that you haven't shown, or your data is something else.

Perhaps use a hash?

#!/usr/bin/perl
use strict;
use warnings;


my $line ="line: label1=N,label2=N,label3=N,label4=NNNNNNNNNNN,label5=
+N,label6=WWWN";

my %values = $line =~ /(\w+)=(\w+)/g;

foreach my $label (sort keys %values)
{
    print "$label $values{$label}\n";
}

__END__
label1 N
label2 N
label3 N
label4 NNNNNNNNNNN
label5 N
label6 WWWN
[download]

Why don't you print $line if not defined $data; ?

Even better use Data::Dumper to check for unexpected characters.

Perhaps a regex is better?

#!/usr/bin/perl
use strict;
use warnings;
$|=1; #turn of buffering for STDERR


while (my $line = <DATA>)
{
   next unless $line =~ /\S/; #skip blank lines
   
   my ($label, $data) = $line =~ /\s*(\w+)\s*=\s*(\w+)/;
   die "bad line: $line" unless defined $data;
   
   print "label $label is: $data\n";
   
}

=prints
label a is: 3
label b is: 4
label c is: 55
label d is: 99
bad line: x=

Process completed with exit code 255

=cut


__DATA__
a = 3

b=4
  c = 55       
  d=99
x=
[download]

I was about to make the same suggestions as Corion did, only I am typing slower. Here's one more possible input: If you have an empty input line, i.e. a line which contains only a newline "\n", you get the same warnings.

And, of course, you should closely inspect chunk 2147 of your input file.

As pretty much everyone else has said now the problem is in your data - the stuff you are not showing us. See How to ask better questions using Test::More and sample data for rationale and instructions to produce an SSCCE like this one.

use strict;
use warnings;

use Test::More tests => 1;
use Test::NoWarnings;

my $line = 'foo=a,bar=3,baz=Fizzlegrapplebaum';
my @data = split(',', $line);
foreach my $datapair (@data) {
    my ($label, $data) = split('=', $datapair);
    my $out = "label: $label; data: $data\n";
}
[download]

Of course, here the test succeeds because the initial data is good. Your task is to make this test (or one very similar to it) fail by supplying the duff value for $line.

Run my code and see what it does.
Try this as an alternative:

foreach my $datapair (@data) {
   my ($label, $data) = split('=', $datapair);
######
   die "Bad Line: $datapair" if !defined ($data);
#####
   print "label: $label; data: $data\n";
   print OUT1 "$data;";
}
[download]

Then please show the content of the @data array using Data::Dumper.

please show a few lines of the input file that demonstrates the problem.