$s is only useful in the for loop:

sub validate
{
    my ($p,$s_no) = @_;
    for (my $s = 0; $s<=$s_no; $s++)
    {
        my $command = $CFG::CFG[$s]->{'CMD'};
        if ($command =~ /cd /)
        {
            $$p = $$p - 1;
        }
    }
}
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