Tested with AS 5.8.9, should work (but untested) with pre-5.8:

 
>perl -wMstrict -le
"my $s = qq{line 1\nline 2\nline 3\nline 4\n};
 my $l = 2;
 my @sra = $s =~ m{ (\A (?: [^\n]* \n){$l}) (.*) }xms;
 use Data::Dumper;
 print Dumper \@sra;
"
$VAR1 = [
          'line 1
line 2
',
          'line 3
line 4
'
        ];
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