use Data::Dumper;
my $prev_data;
while (my $data = $sth->fetchrow_arrayref()) {
  if( $prev_data
    && $data->[0] == $prev_data->[0]
    && $data->[1] == $prev_data->[1]
    && $data->[2] == $prev_data->[2]
  ){
    print $data->[4];
  }else{
    print Dumper $data;
  }
  $prev_data = $data;
}
[download]

• Your sample select statement is only select'ing one column, but your code clearly expects at least 5.
• Consider using fetchrow_hashref() instead for clarity (to the reader, ->[0] doesn't mean anything, but ->{last_name} does).
• You may need to use the eq operator instead of == in some/all of your comparisons
• There's probably a pure sql way to do whatever you're trying to do .... maybe something like (can't given exact sql w/o exact details):

  SELECT t.col0, t.col1, t.col2, t.col4
  FROM mytable as t
    JOIN (
      SELECT col0, col1, col2, min(col4)
      FROM mytable
      GROUP BY col0, col1, col2
      HAVING COUNT(*) > 1
    ) as tmp ON
      tmp.col0 = t.col0
      AND tmp.col1 = t.col1
      AND tmp.col2 = t.col2
      AND tmp.col4 <> t.col4
  [download]

  Update: I just noticed the note "I have no control over the select statement that is issued" .. why not? Is the prepare statement you listed not representative of your actual code?

First off.. apologies for not stating the issue more clearly. This was my first post so in future I'll try and be more specific.
The select statement I put in was purely as an example to demonstrate the issue. Granted I should have selected multiple columns. The real statement I'm dealing with is about a page of A4 and contains some pretty horrendous sql (imo). Anyhow suffice it to say the select being issued is fixed.
The number of columns actually being selected is 15 but I'm only interested in a comparison of some of these columns.
Thanks for the answer by the way

my $prev;
my $data;

while ( $data = $sth->fetchrow_arrayref )  
{
    conditional_print ($data, $prev);
    $prev = $data;
}

#repeat for last record
conditional_print ($data, $prev);

$sth->finish;
$dbh->disconnect;

sub conditional_print
{
    my $data = shift;
    my $prev = shift;
    
    if 
    ( 
        $data->[0] eq $prev->[0] && 
        $data->[1] eq $prev->[1] && 
        $data->[2] eq $prev->[2] 
    )
    {
        print $data->[4], "\n";
    }
    else
    {
        print join ("\t", @{$data}), "\n";
    }
}
[download]

#!/usr/bin/perl -l

use strict;
use warnings;

my @old=split ' ', <DATA>;
print "@old";

while (<DATA>) {
    my @cur=split;
    print "@cur[0,1]" eq "@old[0,1]" ?
      $cur[2] : "@cur";
    @old=@cur;
}

__END__
foo bar baz
foo fred  baz
foo  fred burz
bar baz burp
[download]

Additional checks and adaptation to current situation are left as an exercise to the reader.

Although it should be clear that this is meant as a minimal example, I'd like to stress that "@cur[0,1]" eq "@old[0,1]" is a simple minded equality test based on joining on $", which happens to be a space by default. Of course this works here, but is bound to be unreliable in a realistic situation, and you may have to roll a more sophisticated test yourself.