Well for one, correct your syntax. The fifth line should be something like

$comment =~ s/$l/****/g;
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= isn't the right operator for substitution.

Here is some working code that you should be able to modify.

my $foo = "man, that's some badass motherfucking shit!";
my @badwords = qw/ass fuck shit/;

foreach my $word (@badwords) {
    my $bleep = "*" x length($word);
    $foo =~ s/$word/$bleep/g;
}

print $foo, "\n";
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which outputs

man, that's some bad*** mother****ing ****!
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Updated Formatting

Update 2 In your database, the words are one per line it seems. Therefore, the words all have a \n on the end of them, so unless the "bad" word in the comment has a newline after it, it won't match. Make the syntax correction I noted above, and add chomp($l); as the first statement in your loop, and it should work. The chomp removes the last character of a string if it is a newline.