I have verified that your code crashed perl v5.8.8.
This code does seem to run however.

#!/usr/bin/perl
use threads;
use Thread::Semaphore;

sub bad
{
    sub mysub
    {
        print "my sub : $1 $2\n";
    }
    $a = "Hi there and hi there.\n";
    $a =~ s/(\w+ \w+)(?{mysub();})//gi ;
}

sub mythread
{
    print "begin\n";
    print "end\n";
}

my $thr = threads->new(\&bad);
@r=$thr ->join();

my $thr = threads->new(\&mythread);
@r=$thr ->join();

__END__
[download]

Also $a =~ s/(\w+ \w+)/&mysub()/ge; seems to give the same output as your code and also does not seem to cause the crash. You do need to worry about the output of the function though.

#!/usr/bin/perl
use threads;
use Thread::Semaphore;

sub mythread
{
    print "begin\n";
    print "end\n";
}

sub mysub
{
    print "my sub : $1 $2\n";
    '';
}
$a = "Hi there and hi there.\n";
$a =~ s/(\w+ \w+)/&mysub()/ge;

my $thr = threads->new(\&mythread);
@r=$thr ->join();

__END__
[download]