sub make_re {
    my $len = shift;
    $len > 3 or die "len <= 3";
    my $re = "\\b"
             . join('|', map("\\w(?:\\s[\\s\\w]{$_,}?", reverse 1 .. ($len - 3)), "\\w+\\s+")
             . (")" x ($len - 3)) . "\\w+";
    warn "re: /$re/\n";
    return qr/($re)/;
}

my $re = make_re(5);
while(<DATA>) {
    print "$1\n" if $_ =~ $re;
}
__DATA__
foo foo
foooooooo
foooooooo fooo
foo foo foo foo foo
foo foooooooo foo foo
foo f foo
fo fo foo
f fo foo
f fo