use strict;
use warnings;

##</code><code>##


use strict;
use warnings;
# Read the values from XREF into a hash
my $xref = "input_file.txt";  # put here real filename
open my $XREF, "<", $xref or warn "Could not open $xref $!";  # A more detailed message can sometimes be useful
my %xreflines =  map { chomp; $_ => 1 } grep /\S/, <$XREF>;     # populating the hash with the content of the xref file
close $XREF;

my $goodfile = ...;                                 # insert the name of the output file here
open my $XFILE, ">>", $goodfile or warn "Could not open $goodfile $!";

my $outfile = ...;                                  # insert the name of the customer file here
open my $CUSTOMERFILE, "<", $outfile or warn "Could not open $outfile $!";
while (my $line = <$CUSTOMERFILE>) {
    my ($dist, $cust) = (split /;/, $line)[0,1];    # no need to chomp the fields, there are other fields afterwards in the line
    my $key = "$dist$cust";                         # Building the lookup key
    unless (exists $xreflines{$key}) {              # hash lookup
        print $XFILE "$line";                       # printing the lines whose ID is not found in the cross ref file
    }
}
close $XFILE;
close $CUSTOMERFILE;