sub index_split_join {
    return $_[0] unless index $_[0], 'STOCK' >= 0;  # do a fast check to see if line needs to be looked at
    my @tokens = split /\|/, $_[0];   # split into columns
    $tokens[15] =~ s/STOCK/BOXXE/;    # do replacement in col 16
    return join('|',@tokens);         # glue back together for final result
}

##</code><code>##

                 Rate    splitjoin idxsplitjoin simple_regex
splitjoin     50000/s           --         -15%         -60%
idxsplitjoin  58824/s          18%           --         -53%
simple_regex 125000/s         150%         112%           --

##</code><code>##

my @lines = <DATA>;
cmpthese(10000, {
    idxsplitjoin    => sub {index_split_join($_) for @lines},
    splitjoin    => sub {split_join($_) for @lines},
    simple_regex => sub {simple_regex($_) for @lines},
});
# RESULTS:
Benchmark: timing 10000 iterations of idxsplitjoin, simple_regex, splitjoin...
idxsplitjoin:  9 wallclock secs ( 9.16 usr +  0.00 sys =  9.16 CPU) @ 1091.70/s (n=10000)
simple_regex: 11 wallclock secs (10.77 usr +  0.00 sys = 10.77 CPU) @ 928.51/s (n=10000)
 splitjoin: 158 wallclock secs (158.15 usr +  0.00 sys = 158.15 CPU) @ 63.23/s (n=10000)
               Rate    splitjoin simple_regex idxsplitjoin
splitjoin    63.2/s           --         -93%         -94%
simple_regex  929/s        1368%           --         -15%
idxsplitjoin 1092/s        1627%          18%           --