I don't know the cause of your problem, but I've had success with the Solaris/Perl/OracleInstantClient/cron combination by including a BEGIN block in my perl code as below:

BEGIN {
  unless (($ENV{BEGIN_BLOCK}) or $^C)
  {
    $ENV{ORACLE_HOME}     = '/usr/local/instantclient';
    $ENV{LD_LIBRARY_PATH} = '/lib:/usr/local/instantclient:/usr/local/
+lib';
    $ENV{TNS_ADMIN}       = '/apps/admin'; 
    $ENV{BEGIN_BLOCK} = 1;
    exec 'env',$0,@ARGV;
  }
}
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Obviously you should set your ORACLE_HOME and LD_LIBRARY_PATH env variables to the correct ones for your installations in the above. Note that the TNS_ADMIN env variable tells you where to look for tnsnames.ora.