BEGIN {
 eval { require Compress::Zlib; import Compress::Zlib 'crc32'; };
 *myCRC = $@
  ? sub { 0 } # don't have it
  : sub { crc32(@_) };
}
[download]

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.