use strict;
use warnings;

my $String = "This is line 1
This is Line 2
This is Line 3
This is Line 4
This is Line 5
This is Line 6
This is Line 7
This is Line 8
This is Line 9
This is Line 10";

print "Last [ 5] Lines.[\n" . Last_N_Lines($String,  5) . "]\n\n";
print "Last [12] Lines.[\n" . Last_N_Lines($String, 12) . "]\n";

sub Last_N_Lines {
    my ($String, $N) = @_;
    my $count = ($String =~ tr/\n//);
    return $String if $count < $N;
    my @Lines = (reverse split /\n/, $String)[0..$N-1];
    return (join "\n", reverse @Lines);
}

__END__
Last [ 5] Lines.[
This is Line 6
This is Line 7
This is Line 8
This is Line 9
This is Line 10]

Last [12] Lines.[
This is line 1
This is Line 2
This is Line 3
This is Line 4
This is Line 5
This is Line 6
This is Line 7
This is Line 8
This is Line 9
This is Line 10]