sub btree
{ #  I personally would much prefer hash reference(s) here
  my ( $disk , $workdir , $outFileName
      , $sep
      , $keyMap_aref
      , $verbose
      , $infile_aref , $indir , $outdir
      , $splitSize
      , $keyPos_aref , $outCol_aref
      ) = @_ ;

  ...

  #  No need for "$pcode" & later "eval $pcode"
  #  Use "$parse->($line)" as you already have
  my $parse =
    make_parse_code( $sep , $splitSize , $keyPos_aref );

  ...
}

sub make_parse_code
{ my ($sep , $size , $items) = @_;
  return
    sub
    { my ($line) = @_;
      join $sep
        , ( (split /$sep/ , $line , $size)[@$items] );
    };
}