use strict;
use warnings;

my @array;

open( my $orig,   '<', 'original_file' );
open( my $backup, '+>', 'original_backup' );

while (<$orig>) {
    push ( @array, $_ );
    print( $backup $_ );
}

close($orig);

open( my $new_orig, '+>', 'original_file' );

print( $new_orig $array[-1] );
print( $new_orig @array );

close( $new_orig );