Can we assume that <nth line> really is just your notation for the beginning of a new line, where lines end with "\n"?

If that's the case:

use strict;    # Don't comment it out.
open RERUN, "rerunlrg" or die "Couldn't open input file.\n$!";
while ( my $line = <RERUN> ) {
    print "Matched:\n$1\n\n" if $line =~ m/^(\S+\s+\S+)/;
}
close RERUN;
[download]

Go ahead and give that a try.

One problem your regexp has is that you're not putting a quantifier after your \w metachars. That means that you're trying to match a single word character, followed by any positive amount of whitespace, followed by a single word character, followed by a single whitespace, followed by any amount of anything. You really want to be matching words of arbitrary length, presumably longer than a single character each.

Another problem your script has is that you're not checking for success when you open the file. You should probably be invoking die if there is a failure to open the file. See the example I provided. You can read up on this issue in perlopentut.