Thanks moritz

Now I understand, because my subroutine does NOT return a number. It makes mathematical operations on files and prints results but there is NOT a return so it is normal "die" is executed !

my subroutine does NOT return a number.

That is not true, according to the documentation for return, and as others in this thread have already pointed out:

(Note that in the absence of an explicit "return", a subroutine, eval, or do FILE will automatically return the value of the last expression evaluated.)

This is why moritz correctly pointed out that your sub is indeed returning a value, and it even might be numeric (0).

Consider the following contrived example:

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

my $result;

$result = foo();
print "result=$result\n";
$result = bar();
print "result=$result\n";

sub foo {print "foo: "}
sub bar {print "bar: "; chdir '/nodir'}
[download]

This prints:

foo: result=1
bar: result=0
[download]

sub "foo" returns the result of the successful print command, and sub "bar" returns the result of the unsuccessful chdir command.