my $data = "first line
second line
third line
last line";

my $data2 = $data;

print "Case 1:\n\n";

foreach (1..8){
        my $line = undef;	#lexical variable!
        $line = $1 if $data2 =~ s/^([^\n]*)\n//; #assign to the lexical

        if (! defined $line && $data2 !~ /\n/){
                $line = $data2;
                $data2 = undef;
        };

        print "SPEC: ($line)($data2)\n";
};

print "\n=======\n\nCase 2:\n\n";

foreach (1..8){
        my $line = $1 if $data =~ s/^([^\n]*)\n//; #make a lexical and assign to it

        if (! defined $line && $data !~ /\n/){
                $line = $data;
                $data = undef;
        };

        print "SPEC: ($line)($data)\n";
};

##</code><code>##

Case 1:

SPEC: (first line)(second line
third line
last line)
SPEC: (second line)(third line
last line)
SPEC: (third line)(last line)
SPEC: (last line)()
SPEC: ()()
SPEC: ()()
SPEC: ()()
SPEC: ()()

=======

Case 2:

SPEC: (first line)(second line
third line
last line)
SPEC: (second line)(third line
last line)
SPEC: (third line)(last line)
SPEC: (last line)()
SPEC: (last line)()
SPEC: (last line)()
SPEC: (last line)()
SPEC: (last line)()