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