my $string= " AB CDE FG HI JKL ";
my $PATTERN= qr/(?! F)(?: \w+){1,3} /;
# (match space-separated words where the first doesn't start with "F")
my $last;
my $end= -1;
while(  $string =~ /(?=($PATTERN))/g  ) {
    my $stop= $+[-1];
    if(  $end < $stop  ) {
        $last= $1;
        $end= $stop;
    }
}
print $last, $/;
__END__
prints " HI JKL " !