Are you sure you want that double referencing of the array? Instead of

my @arr=['rose','orange','green'];
$hash{"first"}=[@arr];
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it makes more sense to have

my @arr = qw(rose orange green);
$hash{"first"}=\@arr;
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To add to this array, simply

push(@{$hash{first}}, qw(red blue));
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In entirety:

#!/usr/bin/perl

use warnings;
use strict;

use Data::Dumper;

my %hash;
my @arr = qw(rose orange green);
$hash{"first"}=\@arr;
push(@{$hash{first}}, qw(red blue));

print Dumper(\%hash);

-----

$VAR1 = {
          'first' => [
                       'rose',
                       'orange',
                       'green',
                       'red',
                       'blue'
                     ]
        };
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Update: for your updated question

$hash{first} = [];
push(@{$hash{first}}, \@arr);
...
@newarr = qw(red blue);
push(@{$hash{first}}, \@newarr);
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