The hints how to make an in_array() function are already in the various answers. This just puts them together

usage: in_array($needle,@haystack)

returns: 0 if not found and the number of instances found otherwise

sub in_array {
  my $value = shift(@_) ;
  my @array = @_ ;
  return scalar(grep({$_ eq $value} @array)) ;
}
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