#!/usr/bin/env perl

use strict;
use warnings;
use Text::CSV;

my $unique_within_field = csv_to_unique_within_field(\*DATA, '|');

print "$_: ", join(', ', @{$unique_within_field->{$_}}), "\n"
    foreach sort keys %{$unique_within_field};

sub csv_to_unique_within_field {
    my($data_fh, $sep) = @_;

    my $csv = Text::CSV->new({});
    $csv->header($data_fh, {sep_set => [$sep // ',']});

    my %found;

    while (my $row = $csv->getline_hr($data_fh)) {
        $found{$_}{$row->{$_}} = undef for keys %$row;
    }

    return { map { $_ => [sort keys %{$found{$_}}] } keys %found };
}

__DATA__
head1|head2|head3
val1|val2|val3
val1|val4|val5
val6|val4|val5
val2|val7|val5
val3|val7|val3