use strict;
use warnings;
use Fcntl qw(SEEK_SET SEEK_CUR SEEK_END);

my $data =<< "END";
head1|head2|head3
val1|val2|val3
val1|val4|val5
val6|val4|val5
val2|val7|val5
val3|val7|val3
END


open my $fh , "<", \$data or die "can't open input file $!";

my $header_line = <$fh>; # get first line
chomp $header_line;
my @headers = split(/\|/,$header_line);
my $num_columns = @headers; # scalar value of an array is number of elements

my $begin_of_data = tell($fh); #save byte position of start of data rows

my %seen;
foreach my $column_nr (0..$num_columns-1)
{
    # read whole file looking only at one column
    
    while (<$fh>)
    {
        chomp;
        my $field = (split(/\|/,$_))[$column_nr];
        $seen{$field}++; # counts num times "seen"
    }
    
    # unique values are those which were only "seen" once
    
    print "UNIQUE VALUES for ",shift @headers,":\n";
    foreach my $field_value (sort keys %seen)
    {
        print "$field_value\n" if $seen{$field_value} == 1;
    }
    
    # reset file pointer to beginning and do everything again for
    # the next column
    
    seek ($fh,$begin_of_data,SEEK_SET);
    %seen=();     # clear the hash for calculating next column's data
    print "\n";
}

__END__
UNIQUE VALUES for head1:
val2
val3
val6

UNIQUE VALUES for head2:
val2

UNIQUE VALUES for head3: