Here is a quote from the current (perl 5.12.2) Getopt::Long:
Warning: What follows is an experimental feature.
Others have pointed out why you may be getting an error. This quote should weigh into your decision as to whether you want to use s{,} or not. I interpret "experimental" as meaning it may not be well-tested or reliable, or even that it may be removed in a future release.

In reply to Re: {,} in GetOptions produces error (experimental) by toolic
in thread {,} in GetOptions produces error by pestcat

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