#!/usr/bin/perl -w
use strict;

my $file_name = "test.txt";
open(my $in,  '<', $file_name) || die "cannot open $file_name for read $!";
open(my $out, '>', "$file_name.tmp") || die "cannot open $file_name.tmp for write $!";

while( <$in>) 
{
   print $out "#" if(/\btest\b/i && !/^#/); #only one # at front of line
   print $out $_;
}
close ($in);
close ($out);

# save copy of original file
rename ($file_name,"$file_name.bak")  || die "problem with rename $!";
# replace original with the modified version
rename ("$file_name.tmp", $file_name) || die "problem with rename $!";

= file test.txt before running program
testing is requrired 
see/for/test 
test/data/istesting 

= file test.txt after running program
testing is requrired 
#see/for/test 
#test/data/istesting