Since perl doesn't have a "look behind"

Actually, Perl has a lookbehind (see perlre). What Perl doesn't have is variable length lookbehind, so if you do want lookbehinds of different lengths, they need to be multiple lookbehinds. Furthermore, the \s* makes it require one more lookbehind. This ought to do it though:

/(?<!a)(?<!the)(?<!game)(?<!\s)\s*cock/
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... though I suspect you really want to ensure word boundaries ... and add some /x whitespace for readability:

/ (?<! \b a )
  (?<! \b the )
  (?<! \b game )
  (?<! \s )
  \s* cock \b
/x
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(But that's just a guess, and in any case beyond your question, so you may want to pretend I did not say that.)