$_ = "foo\n Oops";
print /^(?=.{0,50}$).*foo/i ? "Matches!" : $_;

##</code><code>##

/(?=^.{0,50}\z).*foo/si

##</code><code>##

$str =~ s/(foo)/&do_stuff($str) if length $1 < 50; $1/e;