Not sure what you're asking. Do you want to make a copy of a 2D array? Or if you don't really need a copy, can you just use the reference.

my @arr = (
[1,2],
[3,4],
);
my %hash = ( FirstKey => \@arr );

my $aref = $hash{FirstKey};
my $row0 = $aref->[0];
push @$row0, 99;

use Data::Dumper;
print Dumper \%hash;
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