find & replace a string in perl

[vr786]

Hi monks, i am struggling with editing a file , Iam opening a file and replacing string with a comment(#) at start of that line for example: i have a test.txt file in that there is string called "test" , whenever i found the string test , that line should be commented , here is my code

#!/usr/bin/perl -w
my $file_name = "test.txt";
open(INF,'./$file_name') || die "file could not open\n";
my @line;
while(@line = <INF>) {

        if ( $line =~ s/test/^\#/g) {
               open(INF,'>>$file_name') || die "could not open in writ
+e mode";
                print INF "Matching is done by $line \n";
                close(INF);
        } else {
                print "No match found \n";
        }
        close(INF);
}