#!/usr/bin/perl
use warnings;
use strict;

my $string = 'firstfield second field KNOWNWORD alwaysOneWord then one
+ or more make the last field';

my (@fields) = $string =~ / ^ (\S+) [ ] (.*?) [ ] KNOWNWORD [ ] (\S+) 
+[ ] (.*) /x;
for my $i (0 .. $#fields) {
    print "$i:$fields[$i]\n";
}
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Note that it might not work if KNOWNWORD is one of the words in the second field.