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'}
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This prints:

foo: result=1
bar: result=0
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sub "foo" returns the result of the successful print command, and sub "bar" returns the result of the unsuccessful chdir command.

Also your function doesn't use a 'return' statement so it returns the return value of the last statement which is 'close()' in this case. Close seems to return false which might be caused be a failing open() in the first place. You can't close a file which you couldn't open.

Do you use use strict; use warnings; ?? If not put it in and look for correlated warnings.

Dear mscharrer Thanks a lot for your help

I have taken into consideration what you have said. In fact the last line of my subroutine is:

chdir("$path_to_the_script_directory") or die;
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And I do not have any errors (when I do not use "or die" for process() )

But in my operations of process(), I have used :

while (my $line = <$INFILE>){
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And when I want to print $_ after the loop, I have :

Use of uninitialized value in concatenation (.) or string at
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And when I want to print $_ after the loop, I have

$_ and $line are different variables. The contents of one may or may not reflect the contents of the other.

So, what does the subroutine return? Without an explicit return, subroutines return the value of the last statement, in this case, the return of close $OUTFILE. close returns a false value on error, so your close failed.

And a style nit tangental to the die-ing: 99% of the time you don't want to use & when calling subs because it alters how the call is made (see perlsub for the gory details). Calling process( $element ) will work just fine and you'll be glad you didn't get into a bad habit and have to go chasing down why that line &spoo or warn "Zoikes!\n" is getting passed arguments.

open my $OUTFILE, q{>}, $outfile
  or die "failed to open $outfile: $!";
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On another note, you can use the path and skip the chdir calls. e.g.

my $OUTFILE, q{>}, "$path_to_the_data_directory/$outfile"
  or die "failed to open $path_to_the_data_directory/$outfile: $!";
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Your subroutine should start with

sub process{
    my $outfile= shift;
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