my $stream = 't1 t2 t1 t3 t1 t4';
my @tokens = ('t1, 't2', 't3', 't4');

my $pattern = join('|', @tokens);
my $regex = qr/($pattern)/g;

while ($stream =~ $regex) {
    print "token: $1\n";
}


##</code><code>##

while ($stream =~ /$regex/g) {
    print "token: $1\n";
}

##</code><code>##

while ($stream =~ /($pattern)/g) {
    print "token: $1\n";
}