The code:

 if( $line =~ s/\btest\b/^\#/ig ) {
                print  "$line\n";
        }
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is definately wrong as Corion has been saying.
The bit in brackets after the "if" should be a test with a Boolean answer, whereas you've got a substitute command instead. Also if the substitution ever worked you would not be adding a hash in front of the line but replacing "test" with "^#" so your example line "see/for/test " would become "see/for/^#". You want something like:

 if( $line =~ m/test/ ) {
                $line = "#" . $line;
                print  "$line\n";
        }
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