perl regex matched part as output filename - perl

Question

when I have a simple file like
Ann Math 99
Bob Math 100
Ann Chemistry 92
Ann History 78
I may split it into files per person with
awk '{print > $1}' input_filename
However, when the file becomes complex, it is no longer possible to do so unless I use a very complex regex as a field separator. I find that I can extract output filename with some regex, and the following command seems to be able to do what I want for a test with 5 lines:
sed 5q input_filename | perl -nle 'if(/\[([A-Za-z0-9_]+)\]/){open(FH,">","$1"); print FH $_; close FH}'
but the file is large and the command seems to be inefficient. Are there better ways to do it?
original files are like this:
SOME_VERY_LONG_STUFF[TAG1]SOME_EVEN_LONGER_STUFF
SOME_VERY_LONG_STUFF[TAG2]SOME_EVEN_LONGER_STUFF
SOME_VERY_LONG_STUFF[TAG3]SOME_EVEN_LONGER_STUFF
SOME_VERY_LONG_STUFF[TAG1]SOME_EVEN_LONGER_STUFF
SOME_VERY_LONG_STUFF[TAG3]SOME_EVEN_LONGER_STUFF
...
and I just want to split it into files with name TAG1, TAG2, TAG3..., each file contains and only contains lines in the original file that has the tag in the bracket.
the first line with small modifications:
Nov 30 18:00:00 something#syslog: [2019-11-30 18:00:00][BattleEnd],{"result":1,"life":[[0,30,30],[1,30,30],[2,30,29],[3,30,29],[4,30,29],[5,28,29],[6,28,21],[7,28,21],[8,28,14],[9,28,14],[10,29,13],[11,21,13],[12,21,13],[13,15,13],[14,16,12],[15,12,12],[16,12,12],[17,9,12],[18,9,12],[19,5,12],[20,5,12],[21,3,12],[22,3,12],[23,1,12],[24,1,10],[25,1,10],[26,1,10],[27,1,10],[28,2,9],[29,-1,9]],"Info":[[160,0],[161,0],[162,0],[163,0],[155,0],[157,0],[158,0],[159,0]],"cards":[11401,11409,11408,12201,12208,10706,12002,10702,12207,12204,12001,12007,12208,10702,12005,10701,12005,11404,10705,10705,12007,11401,10706,12002,12001,12204,10701,12207,11404,11409,11408,12201]}
the tag I want is "BattleEnd". I want to split the log according to log sources.

Answer 1

EDIT: Since OP changed samples so adding this code now, completely based on shown samples of OP.
awk -F"[][]" '{print >> ($4);close($4)}' Input_file
OR if you want to close output files(to avoid too many files opened error) on whenever previous field is NOT matched then try following.
awk -F"[][]" 'prev!=$4{close(prev)} {print >> ($4);prev=$4}' Input_file
Could you please try following, based on your shown samples.
awk '
match($0,/[^]]*/){
val=substr($0,RSTART,RLENGTH)
sub(/.*\[/,"",val)
print >> (val)
close(val)
}
' Input_file