Here's a pure-regex approach. It needs Perl version 5.10 or higher due to the use of the  \K construct, but that can be worked around if needed.

c:\@Work\Perl\monks>perl -wMstrict -le
"use 5.010;
 ;;
 my $s = qq{foo\nxyzzy\nline 1\nline 2\nline 3\nline 4\nline 5\nline 6
+\netc.};
 print qq{[[$s]]};
 ;;
 my $line   = qr{ [^\n]* \n }xms;
 my $before = qr{ xyzzy  \n }xms;
 my $n = 4;
 my $match = my ($n_lines) = $s =~ m{ $before \K  ((?: $line){$n}) }xm
+s;
 print qq{match: <<$n_lines>>} if $match;
"
[[foo
xyzzy
line 1
line 2
line 3
line 4
line 5
line 6
etc.]]
match: <<line 1
line 2
line 3
line 4
>>
[download]

Update: Actually, just take the  \K out and this code, as it stands, works just fine under any Perl version!