Answers already abound so if it were for homework I'd be tempted to submit something like the following. That said I once put this in an assignment:

printf("Segmentation fault\n");
sleep(4);
printf("just kidding. it's all good.\n");
[download]

My TA didn't find it as funny as I did. When she was a younger student and working late (ie 2 AM) someone put a similar printf() in someone else's assignment while the person went out for a cigarette. That wasn't so funny either. Now back to the perl question:

sub first_n_chars($$) {

        my $N = int shift;
        my $input = shift;
        my $output = "";

        for(;$N > 0; $N--) {
                $input =~ m/(.)/;
                $output .= $1;
        }
        return $output;
}

sub first_n_chars_2($$) {

        my $N = int shift;
        my $input = shift;

        my $dots = "";
        for(;$N > 0; $N--) { $dots .= '.'; }

        my $output = "";
        $input =~ m/($dots)/;
        $output .= $1;

        return $output;
}

sub first_n_chars_3($$) {

        my $N = int shift;
        my $input = shift;
        my $output = "";

        $input =~ m/(.{$N})/;
        $output = $1;

        return $output;
}

my $foo = "314159";
print "$foo\n";
print first_n_chars(1, $foo) , "\n";
print first_n_chars_2(2, $foo) , "\n";
print first_n_chars_3(3, $foo) , "\n";
[download]

Update: Changed " s/$BLAH//; " to " m/$BLAH/; "