You may think you have, but you made a mistake :) Show your code and I'll tell you where

#!/usr/bin/perl --

use strict;
use warnings;

use Data::Dumper;
my @cols = ( 12, 12, 12 );

print Dumper( \@cols ),"\n";

tr/12/91/ for @cols[1.. $#cols];

print Dumper( \@cols ),"\n";

__END__
$VAR1 = [
          12,
          12,
          12
        ];

$VAR1 = [
          12,
          '91',
          '91'
        ];
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