#!/usr/bin/perl

use strict;
use warnings;

open my $accnts, '<', 'accounts.txt'
  or die "can't open accounts.txt: $!";
  
my %lookup = map {chomp; $_ => undef} <$accnts>;

open my $csv, '<', 'csv.txt'
  or die "can't open csv.txt: $!";

open my $output, '>', 'output.txt'
  or die "can't open output.txt: $!";

while (my $record = <$csv>){
  chomp $record;
  # change to suit the order of the fields
  my ($file, $accnt, undef, undef, undef) = split /,/, $record;
  next if exists $lookup{$accnt};
  print $output "$file -> $accnt\n";
}
  
=pod

accounts.txt
acnt1
acnt2
acnt3
acnt4

csv.txt
fil1,acnt1,col3,col4,col5
fil2,acnt2,col3,col4,col5
fil3,acnt3,col3,col4,col5
fil4,acnt4,col3,col4,col5
fil5,acnt5,col3,col4,col5
fil6,acnt6,col3,col4,col5
fil7,acnt7,col3,col4,col5
fil8,acnt8,col3,col4,col5
fil9,acnt9,col3,col4,col5

output.txt
fil5 -> acnt5
fil6 -> acnt6
fil7 -> acnt7
fil8 -> acnt8
fil9 -> acnt9

=cut