For checking whether a list contains an element matching certain criteria or not, grep is a natural choice (natural for me, anyway). So if you merely need to know whether this retCode occurs, you could do this (based on toolic's script):

#!/usr/bin/perl

use strict;
use warnings;
use feature qw/say/;

my @retCodes_values;

while (<DATA>) {
    next unless /retCode\=/;
    chomp;

    my ($string, $exit_code) = split /retCode\=/, $_, 2;
    push @retCodes_values, [ $string, $exit_code ];
}

if(grep { $_->[1] == 3 } @retCodes_values) {
    say "retCode=3 occurred!"
}

__DATA__
15288 :: Mon Mar 2 13:23:32 2015::App.pm:status returned with retCode=
+1
15288 :: Mon Mar 2 13:24:32 2015::App.pm:status returned with retCode=
+3
15288 :: Mon Mar 2 13:25:32 2015::App.pm:status returned with retCode=
+4
15288 :: Mon Mar 2 13:23:32 2015::App.pm:status returned with retCode=
+5
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If you also need information on e.g. the associated log message, just save grep's result to another array.