http://www.perlmonks.org?node_id=363837

#!/usr/bin/perl use warnings; use strict; open PATIENTS, "PATIENT_TABLE_UPLOAD.csv" or die "problem opening PATIENTS file\n"; my @ACFUnumbers; my @duplicateACFUS; while (<PATIENTS>) { my @fields = split /,/; my $ACFU = $fields[1]; push @ACFUnumbers, $ACFU; } #end while foreach my $outerACFU (@ACFUnumbers) { $outerACFU = shift; foreach my $innerACFU (@ACFUnumbers) { unless (!defined($innerACFU) or ($innerACFU =~ /^\s$/)) { if (defined ($outerACFU) and defined($innerACFU)) { push @duplicateACFUS, $outerACFU if ($outerACFU eq $innerACFU); } #end if } #end unless } #end inner foreach } #end foreach print "$_\n" foreach @duplicateACFUS;