For a quick check this might do?

perl -MData::Dump=pp -nle"push @{ $h{ $_ } }, $ARGV }{ pp\%h" 30.txt 4
+0.txt
{
  X0001 => ["30.txt", "40.txt"],
  X0002 => ["30.txt", "40.txt"],
  X0003 => ["30.txt", "40.txt"],
  X0004 => ["30.txt", "40.txt"],
  X0006 => ["30.txt", "40.txt"],
  X0007 => ["40.txt"],
  X0008 => ["30.txt", "40.txt"],
  X0009 => ["30.txt"],
  X0010 => ["30.txt", "40.txt"],
  X0011 => ["30.txt", "40.txt"],
  X0012 => ["30.txt", "40.txt"],
  X0013 => ["30.txt", "40.txt"],
  X0014 => ["30.txt"],
  X0015 => ["30.txt", "40.txt"],
  X0016 => ["30.txt", "40.txt"],
  X0017 => ["40.txt"],
  X0018 => ["30.txt", "40.txt"],
  X0019 => ["30.txt", "40.txt"],
  X0020 => ["30.txt"],
  X0021 => ["40.txt"],
  X0022 => ["40.txt"],
  X0023 => ["30.txt", "40.txt"],
  X0024 => ["40.txt"],
  X0025 => ["30.txt", "40.txt"],
  X0026 => ["30.txt", "40.txt"],
  X0027 => ["30.txt", "40.txt"],
  X0028 => ["40.txt"],
  X0029 => ["30.txt", "40.txt"],
  X0030 => ["30.txt"],
  X0031 => ["40.txt"],
  X0032 => ["30.txt", "40.txt"],
  X0033 => ["40.txt"],
  X0034 => ["30.txt", "40.txt"],
  X0035 => ["30.txt"],
  X0038 => ["40.txt"],
  X0039 => ["30.txt"],
  X0040 => ["40.txt"],
  X0041 => ["40.txt"],
  X0043 => ["40.txt"],
  X0044 => ["40.txt"],
  X0045 => ["30.txt", "40.txt"],
  X0046 => ["40.txt"],
  X0047 => ["30.txt", "40.txt"],
  X0048 => ["40.txt"],
  X0049 => ["40.txt"],
  X0050 => ["30.txt", "40.txt"],
}
[download]

Perhaps an improvement:

perl -nlE"push@{$h{$_}},$ARGV}{say qq[@{$h{$_}}:$_] for keys%h" 30.txt
+ 40.txt | sort
30.txt 40.txt:X0001
30.txt 40.txt:X0002
30.txt 40.txt:X0003
30.txt 40.txt:X0004
30.txt 40.txt:X0006
30.txt 40.txt:X0008
30.txt 40.txt:X0010
30.txt 40.txt:X0011
30.txt 40.txt:X0012
30.txt 40.txt:X0013
30.txt 40.txt:X0015
30.txt 40.txt:X0016
30.txt 40.txt:X0018
30.txt 40.txt:X0019
30.txt 40.txt:X0023
30.txt 40.txt:X0025
30.txt 40.txt:X0026
30.txt 40.txt:X0027
30.txt 40.txt:X0029
30.txt 40.txt:X0032
30.txt 40.txt:X0034
30.txt 40.txt:X0045
30.txt 40.txt:X0047
30.txt 40.txt:X0050
30.txt:X0009
30.txt:X0014
30.txt:X0020
30.txt:X0030
30.txt:X0035
30.txt:X0039
40.txt:X0007
40.txt:X0017
40.txt:X0021
40.txt:X0022
40.txt:X0024
40.txt:X0028
40.txt:X0031
40.txt:X0033
40.txt:X0038
40.txt:X0040
40.txt:X0041
40.txt:X0043
40.txt:X0044
40.txt:X0046
40.txt:X0048
40.txt:X0049
[download]