OK. Thanks Choroba, that makes sense.

However your code didn't work at first, and reading that you said

You need to open the file before you start the loop and close it after you finish reading it.

I tried

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

my $fichier_sortie = 'sortie.txt';
open my $sortie,  '>', $fichier_sortie  or die "Can't open $Fichiersor
+tie: $!";
while (<>){
    chomp;
    my $voir = $_;
    $voir =~ s/o/y/g;
    say {$sortie} $voir;
}
    close $sortie;
[download]

And Yay !