If you'd like to separate the use Package; command from the test, you can still do it.

Use the %INC hash (see perlvar). You need to look for a filename, though, not the package name:

eval { use Compress::Zlib; }
# ... much other code, or in some other module...
BEGIN {
  *myCRC = (exists $INC{'Compress/Zlib.pm'}) ?
    sub { crc32(@_) } : sub { 0 };
}
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