Something like this?

#! perl -slw
use strict;
use threads;
use threads::shared;
use Thread::Queue;

$|++;

sub filler {
    my( $Q ) = @_;
    while( my $ref = $Q->dequeue ) {
        lock $$ref;
        $$ref .= 'X'x10 while sleep 1 and length $$ref < 30;
        cond_signal $$ref;
    }
}

my $Q = new Thread::Queue;
my $buffer :shared;

my $filler = threads->create( \&filler, $Q );

for( 1 .. 5 ) {
    lock $buffer;
    $Q->enqueue( \$buffer );
    cond_wait( $buffer );
    print $buffer;
    $buffer = '';
}

$Q->enqueue( undef ); ## kill thread

$filler->join;
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