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if ($target !~ /..../) {
    print "Syntax Error: Target should be x or y or z\n";
    pod2usage(-verbose => 1);    # just the usage
    exit;
}
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Update

BTW, as I see you are a perl@newbie are you using strict and warnings in your script? If not have a look at Use strict warnings and diagnostics or die to see why it is a good idea.

Cheers,
R.