I can't determine what is generating the error.

You could use eval to trap the errors

#!/usr/bin/perl
use strict;
use warnings;
use CGI ':standard';
use CGI::Carp qw(fatalsToBrowser);
use Time::Piece;

# email addresses
my $dir = '../../public_html/';
open CSV,'<',$dir.'email.txt' or die "CSV $!";
my @csv = <CSV>;
close CSV;
chomp(@csv);

# build one regex
my $re = join '|', 
         map { quotemeta } 
         sort { length $b <=> length $a } @csv;
$re = qr/$re/;
         
# seach text file
my %date = ();
my @error=();
my $lineno = 0;

open BUYERS,'<',$dir."buyers.txt" or die "BUYERS $!";
for (<BUYERS>){
  ++$lineno;
  if (/($re)/){    
    my $addr = $1;  
    if (/(\d+\/\d+\/\d+)/){
      my $date;
      if ( eval{ $date = Time::Piece->strptime($1, '%m/%d/%Y') ; 1} ){
        push @{$date{$addr}},$date->strftime('%Y%m%d');       
      } else {
        push @error,"Error parsing '$addr' date : '$1' on line $lineno
+\n";
      }
    }
  }
}

# sort dates
my @output;
for my $addr (@csv){
  push @output,$addr,sort @{$date{$addr} || []};
} 
 
# output 
print "Content-type: text/html\n\n";
print "$_\n" for @error;
print "$_\n" for @output;

[download]
Updated
poj

In reply to Re: time::piece Error parsing time at /usr/lib64/perl5/Time/Piece.pm line 469 by poj
in thread time::piece Error parsing time at /usr/lib64/perl5/Time/Piece.pm line 469 by darkwell

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