Two simple solutions come to mind. First, the one-liner:

perl -pi.bak -e "$_ = '# ' . $_ if /\btest\b/" filename.txt
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Or the Tie::File approach for simple line by line file handling.

use strict;
use warnings;
use Tie::File;

tie my @filelines, 'Tie::File' or die $!;

foreach ( @filelines ) {
    $_ = '# ' . $_ if /\btest\b/;
}

untie @filelines or die $!;
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I know Tie::File isn't sexy in Perl culture, but it gets the job done without much thought.