I cannot use fixed length, I have to use "substr" to find the values I need ...

I don't understand. All the examples of substr that appear in the OPed code use fixed offsets and lengths (at least, I assume you're the AnonyMonk who posted the OP). Sticking with fixed-width fields, here's another approach that might serve your needs — although I'm not really sure what those needs are! See pack for info on template specifiers; the  '@' specifier in an unpack template moves to an absolute position. Note that the  'x' specifier makes relative forward moves if you can figure out the relative displacements needed; this will save some absolute back-and-forthing. Also see perlpacktut.

>perl -wMstrict -lE
"my $data =
   'ACCTAxACCTBxACCTCxxxFOOxxBARxxBAZxxBOFFxxxxx';
 say qq{'$data'};
 ;;
 my ($fr1, $fr2, $fr3,  $at20, $at25, $at30, $at35) =
   unpack '@0 a5  @6 a5  @12 a5  @20 a3  @25 a3  @30 a3  @35 a4', $dat
+a;
 ;;
 say qq{'$fr1' '$fr2' '$fr3'};
 ;;
 my $account = $fr3;
 ;;
 my $value     = ($account eq 'ACCTC') ? $at20 : 'unknown';
 my $end_value = ($value   eq 'FOO')   ? $at25 : $at30;
 ;;
 say qq{account '$account'  value '$value'  end value '$end_value'};
"
'ACCTAxACCTBxACCTCxxxFOOxxBARxxBAZxxBOFFxxxxx'
'ACCTA' 'ACCTB' 'ACCTC'
account 'ACCTC'  value 'FOO'  end value 'BAR'
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Update: Changed example code to simplify logic.