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 )= $string =~ /.*($PATTERN)/sg;
print $last, $/;
$last= ( $string =~ /($PATTERN)/g )[-1];
print $last, $/;
__END__
prints:
 JKL
 JKL
never:
 HI JKL