Your problem is the regex you are using. Firstly, you need to negate the character class as [0-9] is going to match numbers, not the other was round. What you need is [^0-9]. Secondly, you should change the iterator from * (zero or more) to + (one or more). This is because zero or more digits will match at the beginning of the "null" element so you will get a replacement there resulting in "xraynull" and you will also change the "6" in the next array to "xray". Correcting the character class but leaving the iterator ([^0-9]*) will start to do the right thing with "null" but will make a mull of the first element of the next array resulting in "xray6".

Incorporating both changes with [^0-9]+ does the right thing. Refactoring your code to incorporate strictures and create the data structure in one go

use strict;
use warnings;

use Data::Dumper;

my $raStruct = [
   [ qw{null 44 4} ],
   [ qw{6 24 6} ],
   ];

print Data::Dumper->Dump([$raStruct], [q{raStruct}]);

my $test = q{xray};

map { $_->[0] =~ s/[^0-9]+/$test/ } @$raStruct;

print Data::Dumper->Dump([$raStruct], [q{raStruct}]);
[download]

gives the following output

$raStruct = [
              [
                'null',
                '44',
                '4'
              ],
              [
                '6',
                '24',
                '6'
              ]
            ];
$raStruct = [
              [
                'xray',
                '44',
                '4'
              ],
              [
                '6',
                '24',
                '6'
              ]
            ];
[download]

I hope this is of use.

Cheers,

JohnGG