#!/usr/bin/perl 
use strict;
use warnings;  

my $readfile = 'blah.csv';
my $writefile = 'bleh.csv';

# Note that you were opening $writefile for READ
open my $fh, "<", $readfile or die "Unable to open $readfile: $!";
open my $wfh, ">", $writefile or die "Unable to open $writefile: $!";

foreach (<$fh>) { 
    
    $_ = uc $_;
    chomp;
    
    my ($col1, $col2) = split /;/;
    my @col1_words = split /\s+/, $col1;
    my @col2_words = split /\s+/, $col2;
    
    my %hash;
    @hash{@col1_words} = undef;
    
    my $found = 0;
    for my $word (@col2_words) {
        $found++ if exists $hash{$word}
    }
    
    print $wfh "$_;".@col1_words.";$found\n";
    
}

close ($fh);
close ($wfh);