Isn't SvIVX what you're asking for? It's a version of SvIV optimised for the very condition you told us to assume. 
#define SvANY(sv) (sv)->sv_any
#define SvIVX(sv) ((XPVIV*) SvANY(sv))->xiv_iv

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I am just interested to use Perl's data structure for storage.

Then use a string as a C array like BrowserUk suggested, with the following changes:

In reply to Re^3: Need for (XS) speed by ikegami
in thread Need for (XS) speed by spx2

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