# recent perl's:
open( my $fh, '>', \my $scalar ) or die "open failed: $!";
$zip->writeToFileHandle($fh);

# recent & odler perl's:
use IO::Scalar;
# or:
# use IO::String (apparently more recent than IO::Scalar)

my $fh = IO::Scalar->new(\my $scalar);
$zip->writeToFileHandle($fh);