In your code you have just modified the string.So changes will not be affected in a file.
If you want to modify file contents,use the module File::Inplace module.

Example

use strict;
use warnings;
use File::Inplace;
my $editor = new File::Inplace(file=>"filename",suffix=>".bak");
while ($_=$editor->next_line) {
        if(s/(.*)/"welcome"/)
        {
            $editor->replace_line($_);
        }
    }
$editor->commit;
[download]

If we mention suffix then it will create the backup file with the given extension.