Or:

  $string =~ m/\bip\s*v6\b/i
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which will look for it separated from other stuff by something which would not match '\w' (where the start and end of the string are deemed not to match '\w'). This will not match 'hip v6' or 'ip v61' or 'ipv6_1' (the last because '_' will match '\w'). It will match 'ip v6.12' -- but only up to (but not including) the '.'.