Please take the time and reduce your code to the bare minimum. The problem was completely unrelated to ps -ef, it was completely unrelated to the actual platform, it was completely unrelated to the weird inclusion of other scripts you did.

The problem can be reduced to the following code:

use strict;
my $PLATFORM = 'solaris';

foreach($PLATFORM) {
    (/solaris/ || /tru64/ || /linux/ || /aix/) and do {
        my $i=1;
        print "\$PLATFORM is '$PLATFORM'\n";

        open (PSTEST, $0);
        while (<PSTEST>){
                print "Line # ".$i."\n".$_;
                print "Platform now is '$PLATFORM'\n";
                $i++;
        };
        print "All done!\n";
        close PSTEST;

        print "Platform now is '$PLATFORM'\n";
    };
};
[download]

Please note the somewhat more instructive debugging output. Your problem stems from the spurious foreach ($PLATFORM) loop, which sets $_ as an alias to $PLATFORM and then clobbers it in the while loop. You can circumvent the problem by removing the spurious foreach loop which will execute only once anyway.

use strict;
my $PLATFORM = 'solaris';

#foreach($PLATFORM) {
  $_ = $PLATFORM;
    (/solaris/ || /tru64/ || /linux/ || /aix/) and do {
        my $i=1;
        print "\$PLATFORM is '$PLATFORM'\n";

        open (PSTEST, $0);
        while (<PSTEST>){
                print "Line # ".$i."\n".$_;
                print "Platform now is '$PLATFORM'\n";
                $i++;
        };
        print "All done!\n";
        close PSTEST;

        print "Platform now is '$PLATFORM'\n";
    };
#};
[download]

You should really really really rewrite the code as it is an optical mess and many of the code and employed techniques are quite bizarre.