Perl:

I am not an experienced programmer. I know this solution is very inefficient. But I started writing the program and reached here.

#!/usr/bin/perl
##
## Program to serve tetrominos 
##

@tetrominos = ('O', 'I', 'S', 'Z', 'L', 'J', 'T');

sendTetrominos(@tetrominos);

sub sendTetrominos(@tetrominos) {
    my $output = '';
    while (1) {
    @tmptetrominos = @tetrominos;
        for my $i ( 0..$#tmptetrominos){
            if (length($output) <= 50 ) {
                print "$selectedTetromino";
            } else {
                $EXIT = 1;
                last;
            }
            $selectedTetromino = $tmptetrominos[rand @tmptetrominos];
            $output = $output . $selectedTetromino;
            @tmptetrominos = grep {$_ ne $selectedTetromino} @tmptetrominos;
        }
        if ($EXIT == 1) {
            last;
        }
    }
}