merge sort in perl

merge.pl

#!perl
use strict;
use warnings;

sub merge_sort {
    my $target_array_ref = shift;

    my $len = scalar @{$target_array_ref};
    if ($len <= 1) {
        return $target_array_ref;
    } else {
        my ($a_ref, $b_ref) = _split($target_array_ref);
        return merge( merge_sort($a_ref), merge_sort($b_ref) );
    }
}

sub _split {
    my $a_ref = shift;

    if (scalar @{$a_ref} == 1) {
        return $a_ref;
    }

    my $num = int(scalar @{$a_ref} / 2);

    my @rest = splice @{$a_ref}, $num;
    return ($a_ref, \@rest);
}

sub merge {
    my ($a_ref, $b_ref) = @_;

    my ($a_len, $b_len) = map { scalar @{$_} } ($a_ref, $b_ref);
    if ($a_len == 0) {
        return $b_ref;
    } elsif ($b_len == 0) {
        return $a_ref;
    } else {
        my ($a, $b) = ($a_ref->[0], $b_ref->[0]);
        if ($a <= $b) {
            shift @{$a_ref};

            my $c_ref = merge($a_ref, $b_ref);
            unshift @{$c_ref}, $a;
            return $c_ref;
        } else {
            shift @{$b_ref};

            my $c_ref = merge($a_ref, $b_ref);
            unshift @{$c_ref}, $b;
            return $c_ref;
        }
    }
}

my @a;
map { push @a, int(rand(10)) } 1..5;
print "Before sort: [ @a ]\n";
my @sorted = @{ merge_sort(\@a) };
print "After  sort: [ @sorted ]\n";