I rather like this, because I think it is simple to understand and would be easy to maintain:

use strict;
use warnings; 
my ($x, $y) = (0,0); #table axes, x is horizontal, y is vertical
my ($max_x, $max_y) = (0,0);
my $table ; #hash of arrays

while (<DATA>) {
    chomp; 
    #print "$_\n";
    if (  $_ =~ m/^v_(.*)\s*{/ ) {
        $y = 0;
    } elsif ( $_ =~ m/^}$/ ) {
        $max_x = $x if $x++ > $max_x;
    } else {
        $table->[$x]->[$y] = $_;
        $max_y = $y if $y++ > $max_y;
    } 
}

for my $y (0..$max_y) {
    for my $x (0..$max_x) {
        my $item = defined( $table->[$x]->[$y] ) ? $table->[$x]->[$y] 
+: '  ';
        print "$item ";
    }
    print "\n";
}

__DATA__ 
v_x {
x1
x2
x3
x4
x5
x6
x7
x8
}
v_y {
y1
y2
y3
y4
y5
}
v_z {
z1
z2
z3
z4
z5
z6
}
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