my $line="One two three four five";
if($line =~ m/(one)\s(two)\s(three)\s(four)\s(five)/i){
 my $one=$1;
 $one =~ s/ten/ONE/i;   #  <--- no match
 print "1: $one\n";
 print "2: $2\n";
 print "3: $3\n";
 print "4: $4\n";
 print "5: $5\n";
}#if

__END__

1: One
2: two
3: three
4: four
5: five