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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You can do something like this:

BEGIN {
 eval { require Compress::Zlib; import Compress::Zlib 'crc32'; };
 *myCRC = $@
  ? sub { 0 } # don't have it
  : sub { crc32(@_) };
}
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The way this works is:

The BEGIN block forces this to happen at the very beginning of the program. The eval tries to load the package (in this case Compress::Zlib) and import the symbol 'crc32' into the current package. If this process succeeds, the eval will leave a null string in $@.

The *myCRC = $@ ? sub { } : sub { } part sets up the name 'myCRC' in the symbol table of the current package to refer to one of two subroutines that are defined anonymously here.