use strict;
use Encode;

open( I, "string.txt" );
my $string = <I>;
chomp $string;   # or: my ($pre,$fol) = split ' ',$string;
                 # if file has previous and following strings
close I;

my $pattern = decode( 'big5-eten', $string );

# you might need a different character-set name (if so, fix it in three places)
# also, if you are using $pre and $fol, you need to decode each one separately
# (e.g. into $pat1 and $pat2)

my $newversion = "2.0";  # or whatever...

open( I, "<:encoding(big5-eten)", "big_data.txt" );

binmode( STDOUT, ":encoding(big5-eten" );

while ( <I> )
{
    s/($pattern).*?($pattern)/$1$newversion$2/;
# or:  s/($pat1).*?($pat2)/$1$newversion$2/;
# maybe you also need the "g" modifier too?

    print;
}