#!/usr/bin/perl -i.bak

while (<>) {
    s/\bSvIV\b/SvIVX/g;
    s/\bSvNV\b/SvNVX/g;
    s/\bSvPV\b/SvPVX/g;
    s/\bSvIVn/SvIV/g;
    s/\bSvNVn/SvNV/g;
    s/\bSvPVn/SvPV/g;
    print;
}