I guess this is more or less what you're looking for (demonstrated under the Perl debugger):

  DB<14> @array = (['X1','X2','X3','X4' ], ,['','key', ''], ['key', ''
+, '']);

  DB<15> x \@array
0  ARRAY(0x600501580)
   0  ARRAY(0x6005d2818)
      0  'X1'
      1  'X2'
      2  'X3'
      3  'X4'
   1  ARRAY(0x6005015f8)
      0  ''
      1  'key'
      2  ''
   2  ARRAY(0x6004fa8c0)
      0  'key'
      1  ''
      2  ''

  DB<16>  for my $subarrayref ( @array ) {  $subarrayref = [ grep leng
+th $_,  @$subarrayref] }

  DB<17> x \@array
0  ARRAY(0x600501580)
   0  ARRAY(0x6005d2d70)
      0  'X1'
      1  'X2'
      2  'X3'
      3  'X4'
   1  ARRAY(0x600500e60)
      0  'key'
   2  ARRAY(0x600635c28)
      0  'key'
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