$_ = q~
one
one two
one two 3
~;

while( /(one) (two)( 3)?/gs ){
    print 'Got 1 ', $1,"\n" if defined $1;
    print 'Got 2 ', $2,"\n" if defined $2;
    print 'Got 3 ', $3,"\n" if defined $3;
    print "----\n";
}
__END__
Got 1 one
Got 2 two
----
Got 1 one
Got 2 two
Got 3  3
----