This is precisely where my mind was when I read the question, but to be accurate, you'd have to reverse the logic a bit. Because RFC 4291 allows for v4 mapped addresses (IPv4 addresses embedded within the v6 address), searching for a dot may give false positives in rare circumstances. The following corrects that problem unambiguously:

if ( $addr =~ /:/ ) { print "IPv6"; }
else { print "IPv4"; }
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