Well it seems to me that you do have a working solution and to add to the other answers you got:

• Since this is a one-liner and you're not under strict anyway, you don't need my %s;
• You can access environment variables directly in perl through %ENV:

  MYPATH=$(perl -e'print join":",grep!$s{$_}++,split/:/,$ENV{MYPATH}')
  [download]

  (I don't like backticks, be it in Perl or shell!)
• else you may take advantage of -a:

  MYPATH=$(echo $MYPATH|perl -F: -lape'$_=join":",grep!$s{$_}++,@F')
  [download]

However be warned: I'm now more playing golf than answering your actual question...

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