>perl -wMstrict -le
"my @names = qw(FLY_ABC123_01.jpg PG_ABC123_03 SPL_ABC123_02.jpg
   FOO_ABC123_011 SPL_ABC123_021 SPL_ABC123_021.jpg);
 sub sort_123 {
   return
     map  $_->[0],
     sort { $a->[1] <=> $b->[1] }
     map  [ $_, m{ .* _ (\d+) }xms ],
     @_
     ;
   }
 my @sorted = sort_123(@names);
 print qq{'$_'} for @sorted;
"
'FLY_ABC123_01.jpg'
'SPL_ABC123_02.jpg'
'PG_ABC123_03'
'FOO_ABC123_011'
'SPL_ABC123_021'
'SPL_ABC123_021.jpg'