Ohke, so I now have this

open (A1, "<An1.txt") or die "can't open";
open (A2, "<An2.txt") or die "can't open";

my @array1;
my @array2;

while (<A1>)
{
    my $in = $_;
    chomp $in;
    push (@array1, [split(//, $in)]);
}

while (<A2>)
{
    my $in = $_;
    chomp $in;
    push (@array2, [split(//, $in)]);
}

my $length = $#array1;
my $total = 0;
for (my $x = 0; $x <= $length; $x++)                                  
+    
{
    my $smallLength = scalar(@{$array1[$x]});
    for(my $q = 0; $q <= $smallLength; $q++)
    {   
        if($array1[$x][$q] =~ /^(\+|\_)$/ && $array2[$x][$q] =~ /^[A-Z
+a-z]$/)                    
        {
            splice(@{$array2[$x]}, $q, 0, ''); 
        }
        
        if($array2[$x][$q] =~ /^(\+|\_)$/ && $array1[$x][$q] =~ /^[A-Z
+a-z]$/)                    
        {                                                             
+        
            splice(@{$array1[$x]}, $q, 0, ''); 
        }
        $total++;
    }
}
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I am trying to compare and calculate the diffence between to text files. Calculations come later in my code but that is not important. I get error message 'Use of uninitialized value in pattern match (m//)' for the 2 IF statements. Any idea why?