There are some variables that can't be used with my, e.g. $" or the like. In this case, you have to use local.

sub MyFunc {
  local $" = shift;
  my @list = @_;
  print "@list\n";
} # MyFunc
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But always try to use my before using local.

Best regards,
perl -e "print a|r,p|d=>b|p=>chr 3**2 .7=>t and t"