my %hash = (
    '$example' => 'This is an example ',
    '$comment' => 'This is a comment'
);

open (FILE, 'file') or die ("Can't open file: $!\n");

while (<FILE>) {
    chomp;
    if (exists $hash{$_}) {
        print $hash{$_};
    } else {
        print $_;
    }
}

close (FILE);

##</code><code>##

my %hash = (
    '$example' => 'This is an example ',
    '$comment' => 'This is a comment'
);

open (FILE, 'file') or die ("Can't open file: $!\n");

while (<FILE>) {
    chomp;
    foreach $key (keys %hash) {
        s/$key/$hash{$key}/g;
    }
    print $_;
}

close (FILE);

##</code><code>##

my %hash = (
    '$example' => '$comment is a variable',
    '$comment' => 'this is a comment'
);