DB<2> use warnings; ""/3
Argument "" isn't numeric in division (/) at (eval 10)[C:/Perl_64/lib/
+perl5db.pl:646] line 2.
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update

Laurent_R++ for correction: numerator <-> denominator

Since you're checking $divide_by for a true value (Truth and Falsehood), it's likely that $data_to_sum[0]->[$c] is empty, as LanX pointed out (Update before posting: and as you just discovered yourself). I just wanted to point out tip #4 from the Basic debugging checklist: Use Data::Dumper to see what the variables really contain (although personally I prefer Data::Dump):

use Data::Dump;
dd $data_to_sum[0]->[$c], $divide_by;
$data_to_sum[0]->[$c] /= $divide_by if $divide_by;
print $data_to_sum[0]->[$c], "\n";
__END__
("", 123)
0
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Update: Actually, it turns out that using Data::Dump in this case might confuse the issue more, since it has an issue that causes it to silence the warning in question. So in this case, I recommend Data::Dumper, and I would recommend turning on its Useqq option (as it makes issues with control characters in the string easier to spot):

use Data::Dumper;
$Data::Dumper::Useqq=1;
print Dumper($data_to_sum[0]->[$c], $divide_by);
$data_to_sum[0]->[$c] /= $divide_by if $divide_by;
print $data_to_sum[0]->[$c], "\n";
__END__
$VAR1 = "";
$VAR2 = 123;
Argument "" isn't numeric in division (/) at ...
0
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Update 2019-08-17: Updated the link to "Truth and Falsehood".

Hello seki,

I read that you found your answer already to your question, but I think it is interesting to read also perlnumber, integer or even casting the numbers with int.

Related to your question, just a simple example:

#!usr/bin/perl
use say;
use strict;
use warnings;
use Math::Roman qw(roman);
use Scalar::Util qw(looks_like_number);

my @array = ("8", "", 8, "132.8", 132.8, "III", roman('III'));
foreach my $array_element (@array) {
    if (looks_like_number($array_element)) {
    say "Element: ".$array_element." is a number";
    } else {
    say "Element ".$array_element." isn't a number";
    }
}

__END__

$ perl test.pl
Element: 8 is a number
Element  isn't a number
Element: 8 is a number
Element: 132.8 is a number
Element: 132.8 is a number
Element III isn't a number
Element: III is a number
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I used Math::Roman and Scalar::Util. Also this is a frequently asked question perlfaq4/How do I determine whether a scalar is a number/whole/integer/float?.

Similarly asked question A function to determine if a string is numeric.

Hope this helps, BR.

Argl. My fault. As often like on StackO, I have discovered the answer just after posting the question.

After using the debugger, the non-numeric value is on the left side of the expression (it is coming from a sql select and can also be null/empty in some circumstances).

I have discovered the answer just after posting the question.

That's called Rubber Duck Debugging, and it happens to the most of us.

I should start looking out for a rubber monk for my desk...

Cheers Rolf
_{(addicted to the Perl Programming Language and ☆☆☆☆ :)
Je suis Charlie!}

PS: what a splendid merchandising idea to generate income for the monastery! xD

"Confess all your bugs, you mortal hacker!!!"

I don't play with ducks, the colleague that teached me Perl rather shown me the teddy bear's method: brian's Guide to Solving Any Perl Problem.

The best programs are the ones written when the programmer is supposed to be working on something else. - Melinda Varian