While merlyn's article does cover the issue, the short answer/easy solution is to use Storable's dclone (core module since 5.8.0)

#!/usr/bin/perl

use warnings;
use strict;
use diagnostics;
use Storable qw/dclone/;
use Data::Dumper;

sub left_triangle {
    my @ali=map dclone($_), @_;
    my @aoa2;
    foreach (1..@ali-1) {
        pop @{$ali[$_]};
        #push @aoa2, $ali[$_];
    }
    return \@ali;
}

my @xx = ([3],[7,5],[2,4,6],[8,5,9,3]);
print Dumper left_triangle(@xx);
print Dumper \@xx;
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