To clarify, I am showing an example of what I am trying to do, trimmed down and not functional. As you can see, it's a simple object with a separate thread that keeps reading the output from a program. The thread should exit when the variable $self->{active} turns false, for example by running the method 'stop'.

package Thingee;
use threads;
use threads::shared;

# CONSTRUCTOR
sub new {
 local *IN;
 my $self = {};
 $self->{active} : shared = 1;  # SYNTAX ERROR HERE
                                # This variable must be shared between
                                # threads.
 $self->{in} = undef;
 $self->{thread} = undef;
 open(*IN, "a_program" ) or $self->{active} = 0;
 $self->{in} = *IN;
 $self->{thread} = threads->new(\&reader, $self);      # Start thread
 bless($self);
 return $self;
}

# READER (Thread)
sub reader {
 my $self = shift;
 my $fh = $self->{in};
 my $line;
 while ( $self->{active} ) {
  $line = <$fh>;
  print "> $line\n";
 }
}

# METHOD
sub stop {
 my $self = shift;
 $self->{active} = 0;
}

# RETURN TRUE FOR THE USE
1;
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