Here is my regexp approach - note that I have two regexps - one for a single line match, and one for a multiple line match:

use strict;

my $re_single = qr/
    (?:\A|[\r\n]) # either the start of text or newline
    (
        [^\r\n]* # any non-newlines
        (?:\z|(?:\r\n|\n\r|\r|\n)\z) # last newline or eof
    )
/xs;

sub Last_N_Lines {
    my ( $str, $n ) = @_;

    my $nless = ( $n - 1 );

    my $re_multiple = qr/
        (?:\A|[\r\n]) # either the start of text or newline
        (
            (?:
                [^\r\n]*
                (?:\r\n|\n\r|\r|\n)
            ){0,$nless}
        ) # have to enclose above so it all goes into $1
        (
            [^\r\n]*
            (?:\z|(?:\r\n|\n\r|\r|\n)\z) # last nl or eof
        )
    /xs;

    if ( $n > 1 ) {
        if ( $str =~ m/$re_multiple/ ) {
            return( "$1$2" );
        } else {
            return( "" );
        }
    }

    if ( $str =~ m/$re_single/ ) {
        return( $1 );
    } else {
        return( "" );
    }
};


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");
[download]

Outputs:

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

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