use strict;
use threads;
use threads::shared;

my $flag : shared = 1;

sub thread_code {
    while( $flag ) {
        # Do your thread stuff here
        # but make sure it loops every now and then
        # so that you will see the flag change
    }
}

my @threads = map{ 
    threads->new( \&thread_code ) ;
} 1 .. 4;

...

# when you get told to finish by the user
# or otherwise decide to exit.

EXIT{
    $flag = 0; # tell the kids its time to go
    $_->join() for @threads; # Wait for them 
    # and go.
}