sub split_join {
    my $line = shift;
    my @tokens = split /[|]/, $line;
    if ($tokens[15] eq 'STOCK') {
        $tokens[15] = 'BOXXE';
        return join('|',@tokens);
    }
    else {
        return $line;
    }
}

##</code><code>##

use strict;
use warnings;
use Benchmark qw(cmpthese);


my $line = <DATA>;


printf "Original: $line";
printf "   split: %s",split_join($line);;
printf "  simple: %s",simple_regex($line);;

cmpthese(5000, {
    splitjoin    => sub {split_join($line)},
    simple_regex => sub {simple_regex($line)},
});


sub split_join {
    my $line = shift;
    my @tokens = split /[|]/, $line;
    if ($tokens[15] eq 'STOCK') {
        $tokens[15] = 'BOXXE';
        return join('|',@tokens);
    }
    else {
        return $line;
    }
}

sub simple_regex {
    my $line = shift;
    #$line =~ s/^((?:[^|]*\|){15})STOCK/${1}BOXXE/;
    $line =~ s{^
                ( 
                  (?:
                    [^|]*
                    \|
                   ) {15}
                )
                STOCK
              }
              {${1}BOXXE}x;
    return $line;
}
__DATA__
AT0000937503|20060530|||142.708534||GROUP AG|30618720||||OPEN|ISIN|4943402|VSE|STOCK|39600000|0.77320|STOCK|test

##</code><code>##

Original: AT0000937503|20060530|||142.708534||GROUP AG|30618720||||OPEN|ISIN|4943402|VSE|STOCK|39600000|0.77320|STOCK|test
   split: AT0000937503|20060530|||142.708534||GROUP AG|30618720||||OPEN|ISIN|4943402|VSE|BOXXE|39600000|0.77320|STOCK|test
  simple: AT0000937503|20060530|||142.708534||GROUP AG|30618720||||OPEN|ISIN|4943402|VSE|BOXXE|39600000|0.77320|STOCK|test
               Rate    splitjoin simple_regex
splitjoin    4274/s           --         -44%
simple_regex 7576/s          77%           --