#!/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 write mode";
                print INF "Matching is done by $line \n";
                close(INF);
        } else {
                print "No match found \n";
        }
        close(INF);
}