#!/usr/bin/env perl

use v5.26.3;
use warnings;

my @matrix;
my ($col2, $col3) = qw{column2 column3};
my $keep_re = qr{1};
my $fmt = "%d: %-5s %s %s\n";

{
    my $col1;
    push @matrix, [$col1, $col2, $col3];
}

{
    my $col1 = [];
    push @matrix, [$col1->[0], $col2, $col3];
}

{
    my @col1s;
    my $elem1;
    push @col1s, $elem1;
    push @matrix, [$col1s[0], $col2, $col3];
}

{
    my @col1s = (1);
    pop @col1s;
    push @matrix, [$col1s[0], $col2, $col3];
}

{
    my @col1s = (2);
    my $elem1;
    unshift @col1s, $elem1;
    push @matrix, [$col1s[0], $col2, $col3];
}

for (1, 2, 12, 100, 234) {
    push @matrix, [$_, $col2, $col3];
}

say '*** Original:';

my @original = @matrix;

for (0 .. $#original) {
    $original[$_][0] //= 'undef';
    printf $fmt, $_, @{$original[$_]};
}

say '*** Filtered:';

my @filtered
    = grep +(defined $_->[0] and $_->[0] =~ $keep_re), @matrix;

for (0 .. $#filtered) {
    $filtered[$_][0] //= 'undef';
    printf $fmt, $_, @{$filtered[$_]};
}

##</code><code>##

*** Original:
0: undef column2 column3
1: undef column2 column3
2: undef column2 column3
3: undef column2 column3
4: undef column2 column3
5: 1     column2 column3
6: 2     column2 column3
7: 12    column2 column3
8: 100   column2 column3
9: 234   column2 column3
*** Filtered:
0: 1     column2 column3
1: 12    column2 column3
2: 100   column2 column3

##</code><code>##

use warnings;
...
# all warnings in effect here
{
    no warnings 'XYZ';
    # all warnings except XYZ in effect here
}
# all warnings in effect here