since you're accessing a global variable in a subroutine, @a will be appended to each time. you need to create a lexical variable, and return it from the subroutine. something like:

sub separate_data
{
  my( @important_data )= @_;
  my @important_results= split / /, @important_data;
  return @important_results;
}

## call it like
my @answer= separate_data( $data[$a] );

## or in a loop as:
for my $a ( 0..10 )
{
  my @answer= separate_data( $data[$a] );
}
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~Particle *accelerates*