Most likely, the OSTYPE environment variable is simply not set, and hence you get an undefined value for it. You can check that by using:

my $OSTY;
if (not exists $ENV{OSTYPE}) {
    warn "Couldn't determine OSTYPE";
    $OSTY = "<unknown>";
} else {
    $OSTY = $ENV{OSTYPE}
};
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More likely though, you might want to inspect the $^O variable, which returns you the type of operating system:

print "Running under $^O";
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You could combine the two techniques, and fall back if $ENV{OSTYPE} is unavailable:

my $OSTY;
if (not exists $ENV{OSTYPE}) {
    warn "Couldn't determine OSTYPE from environment - using $^O";
    $OSTY = $^O;
} else {
    $OSTY = $ENV{OSTYPE}
};
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